The Constitution, Invasion, Immigration, and the War Powers of States

A. The Subject

Recent events at the southern border of the United States have raised controversy about whether, and to what extent, states may respond without federal cooperation. Central to the controversy are two constitutional questions: (1) Upon ratification of the Constitution, did any state sovereign war powers survive, or was all such authority ceded to the federal government? and (2) if any state war powers did survive, what is their scope?

Thus far, scholarship and Supreme Court jurisprudence have provided no clear answers to those questions. 4 This article tackles them.

B. Background Information: The British Empire and Our Sources

Nearly all the leading Founders had been born and raised under the British flag—either in the North American colonies, Britain, Ireland, or (as in the case of Alexander Hamilton) the British Caribbean. Understanding the Constitution they adopted requires some information on the empire they had inhabited.

The island of Great Britain consisted (as it still consists today) of England, Wales, and Scotland. England and Wales had been united for legal purposes in the sixteenth century. The English and Scottish Crowns were conjoined upon the accession of James I in 1603, but England and Scotland remained separate kingdoms, each with its own parliament. Then in 1707, both parliaments passed Acts of Union, thereby creating the Kingdom of Great Britain with a common British Parliament. Within those limitations, Scotland retained its own legal system, as it does today. 5

After the territorial losses from the American Revolution, the Empire encompassed the following territories: the island of Great Britain along with small nearby islands, Ireland, Canada, much of India, Bermuda, an incipient colony in and near Australia, and valuable Caribbean islands, including the Bahamas, Jamaica, and Trinidad.

Most colonies enjoyed at least some degree of self-governance, but they usually fashioned their institutions from English (rather than Scottish or Irish) models. Some core legal concepts (such as “allegiance,” discussed below in Part IV), were common to the entire empire.

As might be expected, the Constitution’s language and structure were influenced heavily by English jurisprudence. 6 One subdivision of that jurisprudence was the law of nations, which today we call international law. A subdivision of the law of nations was the law of war. For information on the law of nations, including the law of war, English lawyers, judges, and commentators relied principally on a handful of authoritative European treatises, 7 as well as on their own legal precedents.

To assist in reconstructing the Constitution’s meaning, we draw heavily on the European “law of nations” treatises and on Anglo-American case reports, law dictionaries, digests, and other legal works used by Founding-era lawyers. We also draw on contemporaneous lay dictionaries and other literary sources.

Courts and lawyers typically refer to Article I, Section 10 of the Constitution as the Compact Clause and Article IV, Section 4 as the Guarantee Clause. Our examination, however, focuses only on selected components of those two provisions. To increase precision, we identify the relevant components as follows: The Self-Defense Clause is the part of the Compact Clause that provides, “No State shall, without the Consent of Congress . . . keep Troops, or Ships of War in time of Peace . . . or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay. 8 The Protection From Invasion Clause is the part of the Guarantee Clause that provides, “The United States . . . shall protect each of them [i.e., the states] against Invasion.” 9 The Domestic Violence Clause is the segment of the Guarantee Clause that reads, “The United States . . . shall protect each of them [i.e., the states] . . . on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.” 10

A. Definitions and Categories of War

The Founders’ international law authorities recognized that the term “war” could describe episodes of combat, but for legal purposes they defined it as a continuous state or condition. Hugo Grotius defined war as “the State or Situation of those . . . who Dispute by Force of Arms.” 11 Emer de Vattel described it as “that state in which a nation prosecutes its right by force.” 12 For a state of war to exist, actual fighting was not necessary. 13

Wars were classified as private, public, or mixed. 14 A private war was prosecuted solely by private parties. 15 Purely private conflict was a subject for natural law or ordinary civilian law, not for the law of nations. 16 In a public war all contending parties were sovereigns. 17 Mixed war was a clash between a sovereign and private persons, 18 such as international criminals of the kind denominated “enemies of the human race.” 19

A war could be offensive and just, offensive and unjust, defensive and just, or—in rare cases—defensive and unjust. 20 The mark of a just war was that it was a final resort for preventing, obtaining compensation for, or avenging injury. 21 Aggression for the sake of gain, conquest, or glory was unjust. 22

A defensive war was one waged to prevent injury. 23 Usually a party engaged in defensive war was not the first to strike, but defensive war could include a preemptive strike to forestall an imminent assault. 24 A party also engaged in defensive war if he attacked because he was “often alarm’d and harass’d with sudden Incursions upon him, the Enemy retiring always when he appears to oppose him.” 25

Offensive wars were fought to seek compensation for perceived injury or to deter the enemy from inflicting anticipated injury. 26 For an offensive war to be considered lawful, those motivations were necessary; otherwise, the attack was unlawful—akin to robbery—and a nation assaulted in that way was not obliged to observe the rules of war in fighting off the assailant. 27

Under the law of nations, only a sovereign was privileged to make war or to delegate the power to do so. 28 The sovereign designated the precise officials empowered to begin a war, who might be agents of subordinate units of government. 29 Even without an express authorization, the governor of a political subdivision had implied authority to defend against invaders or insurrectionists. 30 He was not, however, “rashly to carry the War into an Enemy’s Country.” 31

Initiation of hostilities might be signaled by a declaration of war—sometimes called a “denunciation,” after denuntio, the Latin word for a declaration of war. A declaration was not required for a defensive war, but was expected for an offensive one. 32 Hostilities supported by a declaration were referred to as “formal” or “solemn,” from the Latin solemnis, a word associated with ceremony. 33

B. The Means of War

A just war empowered the sovereign to undertake nearly all means necessary to accomplish its purpose of preventing or repairing injury or forestalling future injury. 34 (“Nearly all means” because some, such as assassination and poisoning, were prohibited by the law of war.) 35 Vattel wrote of defensive conflicts:

The enemy attacking me unjustly, gives me an undoubted right of repelling his violences; and he who opposes me in arms, when I demand only my right, becomes himself the real aggressor by his unjust resistance . . . For if the effects of this force proceed so far as to take away his life, he owes the misfortune to himself; for if by sparing him I should submit to the injury, the good would soon become the prey of the wicked. Hence the right of killing enemies in a just war is derived; when their resistance cannot be suppressed, when they are not to be reduced by milder methods, there is a right of taking away their life . . . . But the very manner by which the right of killing enemies is proved, points out also the limits of this right. On an enemy’s submitting and delivering up his arms, we cannot with justice take away his life. 36

Besides killing enemies who refuse to surrender their arms, a belligerent could capture them, 37 hold them for ransom, 38 make reprisals in certain circumstances, 39 execute war criminals, 40 and seize enemy property. 41 The belligerent could seek out enemies in their territory, in its own territory, or in areas belonging to no one. 42 It could prosecute for treason any of its own subjects caught assisting the enemy. 43 The belligerent also could take many defensive measures that are characteristic of war but which by themselves would fall short of (or be incidental to) full-blown hostilities, such as building protective barriers. 44

Eighteenth century war was often a brutal exercise 45—far more so than the relatively controlled conduct of both sides during the American Revolution. 46 International law scholars, among others, sought to curb the brutality. 47 Their writings encouraged belligerents to exercise mercy and restraint whenever possible, 48 and to transport and release enemies in safe locations. 49

These authorities on the law of nations also laid down the rule that a belligerent should not pursue, seize, or kill enemies in a neutral country. 50 This rule was heavily qualified both in theory and practice. A nation aspiring to neutral status had to “shew [sic] an exact impartiality between the parties at war” 51 and not grant to one quarrelling party what it withheld from the other. 52 A neutral nation could not permit its citizens to injure one of the belligerents by, for example, encroaching over its borders. 53 Even if a country met those standards, a belligerent still might legitimately intrude on neutral territory in cases of extreme necessity, so long as the belligerent later provided compensation. 54 A belligerent also could intrude on neutral territory if the enemy regularly fled into that territory or deposited spoil or prisoners there. 55

C. “Enemies of the Human Race”

Founding-era international law identified persons engaged in particularly reprehensible activities outside ties of national allegiance as “enemies of the human race”—hostes humani generis. 56 They included pirates (defined in eighteenth century dictionaries as “sea robbers”) 57 and other thieves; deserters; 58 poisoners, assassins, and incendiaries; 59 those who participated in combat merely for depredation; 60 and foreigners who were “unauthorized voluntiers [sic] in violence.” 61 Modern analogues include international freelance terrorists and international criminal organizations, such as the Mexican drug and human trafficking cartels. 62

Wars against enemies of the human race were always just. 63 Enemies of the human race could be attacked wherever they happened to be, even if they had not crossed any international boundary. As Vattel remarked:

[I]f the justice of each nation ought in general to be confined to the punishment of crimes committed in its own territories; we ought to except from this rule, the villains, who by the quality and habitual frequency of their crimes, violate all public security, and declare themselves the enemies of the human race. Poisoners, assassins, and incendiaries by profession, may be exterminated wherever they are seized…. 64

A nation capturing enemies of the human race had the choice of treating them as prisoners of war or as common criminals. William Blackstone argued for their being treated as criminals rather than as prisoners of war in the first volume of his Commentaries. 65 In the second volume, however, he implied that civilian-style due process was not required:

As, therefore, he has renounced all the benefits of society and government, and has reduced himself afresh to the savage state of nature, by declaring war against all mankind, all mankind must declare war against him; so that every community hath a right by the rule of self-defence, to inflict that punishment upon him which every individual would in a state of nature have been otherwise entitled to do, for any invasion of his person or personal property. 66

Treating captured hostes humani generis as accused criminals denied them the honorable status of prisoners of war normally accorded captured enemy aliens. Treating them as captured enemy aliens, on the other hand, denied them privileges—such as trial by jury—to which accused criminals were entitled.

D. Allegiance—Cross Reference

The concept of “allegiance” also defined the scope of permissible conduct during war. This subject is addressed in Part IV.

A. Preliminary Comments

The charters of the North American colonies typically granted them authority to wage defensive war. For example, the 1629 royal charter for Massachusetts Bay colony provided in part:

AND WEE [i.e., the king] DOE further . . . give and graunte to the said Governor and Company, and their Successors, by theis Presents, that it shall and maie be lawfull . . . to incounter, expulse, repell, and resist by Force of Armes, as well by Sea as by Lande, and by all fitting Waies and Meanes whatsoever, all such Person and Persons, as shall at any Tyme hereafter, attempt or enterprise the Destruccon, Invasion, Detriment, or Annoyaunce to the said Plantation or Inhabitants . . . 67

When the Declaration of Independence was issued, the thirteen colonies signing the document became states. They thereby assumed as a matter of sovereign right what previously had been a subject of grant. Thus, under both the Articles of Confederation and the Constitution, the source of most state authority 68—including that pertaining to war—preceded the Union and was largely reserved to the states. 69 The provisions in the Articles and the Constitution addressing state war powers served only as limitations or descriptions, not as grants. By contrast, the source of federal authority is the Constitution’s enumeration of powers. 70

To be sure, the controversial “doctrine of inherent sovereign authority” holds that the states never enjoyed power over military and other foreign affairs subjects, and that the federal government received that authority directly from its congressional predecessors—thereby bypassing the Articles and the Constitution entirely. 71 As one of us recently demonstrated, however, this thesis is fatally flawed on every level: historically, legally, and logically. 72 In this article, therefore, we do not address it further.

B. War Powers Under the Articles of Confederation

As the North Atlantic Treaty was to do 168 years later, 73 the Articles of Confederation deputized a central authority with certain prerogatives and limited the signatories accordingly. The rules pertaining to war powers were laid out in Articles VI 74 and IX. 75 The text of the two articles was somewhat disorganized, but it laid out a coherent scheme in which Congress received general authority to declare and wage war for the Confederation. State war powers were reserved but limited in the following respects: 76

• - Congress could set a maximum on the number of naval vessels states could maintain in time of peace;

• - Congress could, upon review, limit the number of state vessels during a state war against pirates;

• - states could grant commissions to ships and vessels of war and issue letters of marque and reprisal only after a congressional declaration of war and only against the declared enemy;

• - states were required to maintain “a well-regulated and disciplined militia, sufficiently armed and accoutered . . . and constantly . . . ready for use;”

• - a state was not to engage in war unless “actually invaded 77 by enemies, or shall have received certain advice of a resolution being formed by some nation of Indians to invade such State, and the danger is so imminent as not to admit of a delay till the United States in Congress assembled can be consulted.”

The upshot was that the states retained virtually unlimited flexibility to engage in defensive land war—even after Congress had been consulted—except for power to strike preemptively at non-Indian enemies. Their naval scope was more constricted: They could maintain navies to fight congressionally-declared wars. They could issue letters of marque and reprisal only against congressionally-declared enemies. They could maintain fleets and launch them to suppress pirates, although limited by congressional review.

As for other powers related to war, the states retained authority to limit foreign immigration, impose embargoes, and suspend the writ of habeas corpus. However, state treaties and alliances were subject to congressional review, and state imposts and duties had to be consistent with congressional treaties. 78

C. Federal War Powers Under the Constitution

Founding-era international law scholars acknowledged each nation’s prerogative of dividing war powers among different administrative levels. 79 The Constitution divided war powers between the federal government and the states by granting authority to the federal government and limiting the reserved authority of the states.

The Protection From Invasion Clause and the Domestic Violence Clause imposed duties on the federal government to wage defensive war under certain circumstances: “The United States . . . shall protect each [state] . . . against Invasion; and on Application of the Legislature, or of the Executive (when the Legislature cannot be convened) against domestic Violence.” 80 The mandates were addressed to the United States government as a whole rather than solely to any branch. 81

The Take Care Clause 82 similarly mandated the President to “take Care that the Laws be faithfully executed.” This was another authorization to wage defensive war.

In addition, the Define and Punish Clause deputized Congress to “define and punish Piracies and Felonies committed on the High Seas.” 83 This permitted “mixed wars” against pirates and any other nautical “enemies of the human race.” Finally, the Constitution granted Congress power to “declare War.” 84 This enabled Congress to fight both defensive and offensive wars, both public and mixed 85—although declarations of war were associated primarily with offensive rather than defensive operations.

Other enumerated powers granted Congress the means to wage war. Congress could:

• - “grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water;” 86

• - “raise and support Armies” 87 and “provide and maintain a Navy;” 88

• - “make Rules for the Government and Regulation of the land and naval Forces;” 89

• - “provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions;” 90

• - “provide for organizing, arming, and disciplining, the Militia, 91 and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress.”

In addition, the Constitution granted Congress and the President certain powers wholly or partly associated with war. Specifically, the Constitution—

• - conferred on the President, with the advice and consent of the Senate, sole authority to make treaties; 92

• - designated the President as “Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States;” 93

• - implicitly granted Congress, as a traditional incident of war-making, the prerogative of suspending the “Privilege of the Writ of Habeas Corpus . . . when in cases of Rebellion or Invasion the public Safety may require it,” 94 thus authorizing suspension during certain defensive, but not offensive, operations;

• - granted Congress authority to “regulate Commerce with foreign Nations,” 95 which enabled it to override certain state measures related to war, such as embargos and other trade restrictions 96 and those governing commercial immigration, including the slave trade; 97 and

• - granted Congress power to “define and punish . . . Offenses against the Law of Nations.” 98 This provision permitted Congress to enact statutes protecting diplomats, fixing protocols of international practice, and restricting non-commercial immigration and emigration. 99 Of course, this clause, like other grants in the Constitution, carried with it incidental powers, recognized under the Necessary and Proper Clause. 100

D. State War Powers Under the Constitution

To the extent the Constitution did not qualify them, war powers remained in the states by reservation. 101 The ratifiers understood this, as demonstrated by the proceedings of the Virginia ratifying convention. At one point, the discussion turned to the Constitution’s grant of power to Congress to

provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States, reserving to the States respectively, the Appointment of the Officers, and the Authority of training the Militia according to the discipline prescribed by Congress. 102

The Constitution’s opponents objected that this clause gave Congress exclusive power over state militias. But the Constitution’s advocates pointed out that the opponents were overlooking state reserved powers. The future Chief Justice John Marshall explained:

The State Legislatures had power to command and govern their militia before, and have it still, undeniably, unless there be something in this Constitution that takes it away …. All the restraints intended to be laid on the State Governments (besides where an exclusive power is expressly given to Congress) are contained in the tenth section, of the first article. This power is not included in the restrictions in that section.—But what excludes every possibility of doubt, is the last part of it.—That “no State shall engage in war, unless actually invaded, or in such imminent danger as will not admit of delay.” When invaded, they can engage in war; as also when in imminent danger. This clearly proves, that the States can use the militia when they find it necessary. 103

Marshall’s analysis was reinforced by James Madison 104 and Edmund Pendleton, the convention chairman. 105 George Nicholas also affirmed that the states, “are at liberty to engage in war when invaded, or in imminent danger.” 106 The popular Federalist essayist Tench Coxe made the same point in the public press: “Any state may repel invasions or commence a war under emergent circumstances, without waiting for the consent of Congress.” 107

The Constitution limited and qualified reserved state war powers in several respects. The result was a balance between federal and state prerogatives roughly similar to that under the Articles of Confederation. But in one way the Constitution constricted the states’ war powers further, and in four ways it actually expanded them.

The Articles had permitted states to maintain naval vessels in peacetime up to a congressionally-prescribed maximum. The Constitution provided, “No State shall, without the Consent of Congress . . . keep . . . Ships of War in time of Peace.” 108 Since the Articles gave Congress authority to fix the peacetime maximum at “zero,” the substantive effects of the two restrictions were the same.

The states’ sole loss of war power was on the naval side. This was the Constitution’s removal of their prerogative to issue letters of marque or reprisal against an enemy upon whom Congress had declared war. 109

The increases in state war powers were as follows: First, the Constitution did not require a congressional declaration of war for states to build ships. It required only war de facto, with no requirement that the war be one waged by the federal government. Second, the Constitution deprived Congress of its veto over state naval actions against invading pirates.

Third, on the land side, the Constitution preserved general state control over their militias while providing that “No State shall, without the Consent of Congress . . . keep Troops . . . in time of Peace . . . or engage in War, unless actually invaded, or in such imminent Danger as will not admit of delay.” 110 This limitation omitted the Articles’ contingent requirement of consultation with Congress. 111

Fourth, while the Articles had permitted state preemptive strikes against imminent invasions by Indians only, the Constitution permitted them against all invasions.

The states also retained unmentioned prerogatives sometimes associated with war. As participants in the ratification debates observed, states would continue to have power to suspend the writ of habeas corpus. 112 In addition, the Constitution implicitly recognized that states could continue to control foreign immigration, subject to some federal preemption before 1808 and more extensive preemption thereafter. 113 The Constitution retained state power to impose embargoes, although subject to federal preemption. 114

Some readers may find the conclusion that the states retained significant military authority to be counterintuitive. In part, this may be due to the fact that the states rarely exercise such authority today. In part, also, it may be due to the general conception of the Constitution as uniformly increasing central power.

The truth, however, is more complicated. In negotiating the constitutional re-arrangement, the states sometimes gained as well as lost, and military affairs may not be the only case of this happening. 115 Furthermore, we should not overestimate the extent to which the Constitution increased central power. During the ratification debates, Justice Nathaniel Peaslee Sargent of the Massachusetts Supreme Judicial Court observed that the Constitution conveyed “[v]ery few” more powers than the Articles of Confederation. 116 The more significant difference between the two documents was that, within its sphere, the new federal establishment was a genuine government, rooted in popular consent and able to enforce its power directly on the people. It was not a mere treaty among state legislatures, as the Confederation had been.

Additionally, curbing state prerogatives and strengthening the central power were not the only reasons for the Constitution. The Founders also sought to protect the states, to prevent them from degenerating into monarchy or anarchy, and to improve the quality of their governance. All these policies are evident in the first sentence of Article IV, Section 4. 117

E. Defining “Invaded” and “Invasion”

The words invade and invasion served as triggers for both federal and state defensive war powers. Thus, the Constitution’s Calling Forth Clause empowered Congress to enlist state militias in federal service “to execute the Laws of the Union, suppress Insurrections and repel Invasions.” 118 The Suspension Clause acknowledged congressional power to suspend the writ of habeas corpus in certain cases of rebellion or invasion. 119 The Protection From Invasion Clause imposed a federal obligation to protect states “from invasion.” 120 The Self-Defense Clause confirmed that a state could engage in war if “actually invaded, or in such imminent Danger as will not admit of delay.” 121 The centrality of the words “invasion” and “invaded” renders their constitutional meaning and scope of great importance.

During the eighteenth century, “invasion” and its variants in their broadest sense could include infringements or attacks on rights and privileges—as in the phrase, “The censorship policy was an invasion of the right of free speech.” 122 The context of the words in the Constitution itself, however, demonstrates that their constitutional meaning is less metaphorical and more concrete: “Invasion” is an incursion into home territory by outsiders.

But what kind of incursion? Is the meaning limited to intrusion by a foreign army? Several Court of Appeals opinions have said as much, but on very sparse evidence. 123 Or is the meaning wider? And if wider, how is it circumscribed?

Eighteenth-century dictionaries inform us that when “invasion” and its variants applied to physical intrusions, the scope was not limited to incursions by a foreign army. Among the thirteen Founding-era English dictionaries we examined, only one seemed to limit “invasion” and its variants to formal military operations. 124 The other twelve included formal military operations, to be sure; but they also added definitions comprehending many other kinds of encroachments and intrusions. These definitions appear in the footnote below. 125

The reader may observe that some of these definitions required that an invasion be “hostile.” For that reason, we included in foot^{note 125} the entry for “hostile” from each dictionary employing that word when defining “invasion” or its variants. As those entries show, “hostile” often meant merely “adverse.” Readers may recognize this as the non-military definition preserved in the modern law of adverse possession and in legal phrases such as “hostile takeover” and “hostile witness.” Thus, all we can infer from the requirement of “hostility” is that for an entry to be an invasion it must be unauthorized and uninvited.

Eighteenth-century American political discourse confirms what the dictionaries suggest: the scope of “invasion” and its variants was quite broad.

First: An invasion could be by sea as well as by land. Both the congressional records 126 and participants in the constitutional debates referred to maritime invasions. 127

Second: An invasion need not be incident to actual warfare, nor an operation of war. The Massachusetts Constitution of 1780, for example, spoke of “time of war or invasion” (and it still does). 128

Third: An invasion need not be launched by a formal military force. Participants in the constitutional debates referred to “invasions of barbarous tribes,” 129 “invasion of the savages,” 130 and “hostile invasions of lawless and ambitious men intending . . . to . . . introduce anarchy, confusion, and every disorder.” 131 In Federalist No. 41, James Madison referred to attacks along the Atlantic coast by “licencious [sic] adventurers . . . daring and sudden invaders.” 132 References to invasions by pirates appear in contemporaneous literature. 133

An “invasion” could refer also to uninvited entry by groups of immigrants. 134 Pennsylvanians used that term to describe the essentially peaceful immigration of Connecticut settlers into Pennsylvania’s Wyoming Valley, because the settlers were relying on legal title that the Pennsylvania government did not recognize. 135 Thus, in 1754, Benjamin Franklin wrote a plan “to divert the Connecticut Emigrants from their Design of Invading this Province [Pennsylvania], and to induce them to go where they would be less injurious and more useful.” 136 At the time, the “invaders” had done little more than purchase disputed title. 137 Peace broke down only when the Connecticut settlers sought to defend themselves from local Indians and the Pennsylvania authorities. 138

In 1775, Congress recommended that Connecticut stop sending settlers until further notice. 139 When, in 1783, the Confederation Congress established a court to adjudicate Wyoming Valley land claims, 140 the Pennsylvania legislature responded in resolutions again charging that the unauthorized Connecticut immigration was an invasion:

[I]f Congress should consent to establish courts at the instance of persons not first proving themselves to be included in the description aforesaid, the citizens of this State may be harassed by a multitude of pretended claims at the suit of adventurers or invaders of the State, and in the present instance at the suit of persons who have settled in defiance of the resolution of Congress of the 23 day of December, 1775. 141

The Constitution did not limit invasions to large-scale incursions—an aspect of the document specifically criticized during the ratification debates. 142 Perhaps the framers agreed with Sir William Yonge’s comment in Parliament that “a small Invasion may be as fatal in its Consequences as the most formidable and most successful Invasion at another Time.” 143 The passage of time seems to have confirmed the judgment that an intrusion may be small and still be classified as an invasion: In the 1942 case of Ex Parte Quirin, 144 the Supreme Court characterized a group of only eight Nazi saboteurs as “invaders.” 145

Nor would it seem that “invaders” had to be armed when crossing the border. Even unarmed persons can cause local disruption, and once they cross the border they may acquire arms and defend their position 146 or cause other damage. By way of illustration, the terrorists of September 11, 2001 arrived unarmed, exceeded the scope of their visas, and hijacked three aircraft on U.S. territory and used them to kill thousands of Americans. Under the Constitution’s definition, they qualify as “invaders.”

In Federalist No. 43, Madison justified the broad meaning of “invasion” when discussing the Constitution’s Protection From Invasion Clause: “The latitude of the expression here used, seems to secure each state not only against foreign hostility, but against ambitious or vindictive enterprizes [sic] of its more powerful neighbours.” 147

There were some limiting factors, however. “Invasion” and its variants did not comprehend all unauthorized intrusions. There had to be detriment (loss, harm, or annoyance) beyond the mere fact of intrusion. Franklin’s letter referred to the “injurious” consequences of the unauthorized immigration into his state. 148 The Pennsylvania legislature felt “harassed” by the unauthorized immigrants. Invasion that had not yet occurred but was imminent posed some “danger” 149—risk of detriment 150—against which “defense” was required.

The actual or threatened detriment from invasion could be injury to persons; 151 physical damage, 152 such as that resulting from plundering; 153 or the breakdown of normal processes of law 154 and communication. 155 During the Connecticut invasion of the Wyoming Valley, Pennsylvania president John Dickinson—later one of the Constitution’s more important framers 156—identified another kind of detriment: the Connecticut settlers were occupying land the state otherwise could sell to raise revenue. 157

Did an incursion have to be organized to qualify as an invasion? We found no evidence that prior coordination was necessary. A spontaneous mob might launch an invasion. On the other hand, prior coordination might demonstrate the existence of detriment or quantify the extent of the risk. Coordination also might demonstrate causation—i.e., that the intrusion was responsible for specified injury.

Relying on the premise that no government in the United States has authority to restrict peaceful immigration, some may exclude non-violent mass immigration from the definition of “invasion.” 158 One problem with this conclusion lies in its premise. It overlooks the Constitution’ explicit recognition that individual states may restrict immigration. 159 It also overlooks the Constitution’s grant to Congress of authority to “define and punish . . . Offenses against the Law of Nations,” 160 which encompasses authority over trans-border migration. 161

Professor Ilya Somin is among the few who deny any federal authority to restrain peaceful immigration from nations with which the United States is not at war. He relies 162 largely on James Madison’s 1800 Virginia legislative report on the Alien and Sedition Acts. 163 However, this document is not useful evidence on the question of whether the Constitution grants Congress authority to restrict immigration. For one thing, it focused not on immigration, but on deportation. For another, it was written a decade after ratification, and did not represent any kind of consensus among the Founders; on the contrary, it was highly partisan and its conclusions were disputed hotly. 164

Nor does the substance of the document provide any evidence on whether Congress has power to restrict immigration.

Madison argued that the Constitution gave Congress no authority to deport “alien friends,” and he classified them as such because they had come from countries with which the United States was at peace. 165 But he did not address the fact (because there was no need to) that not all foreigners from friendly countries qualified as alien friends. As explained in Part IV, an alien friend was a person in allegiance to the host country, and a person who entered sovereign territory in defiance of its laws thereby refused allegiance. 166 This rendered him an alien enemy, or (if the sovereign preferred) rendered him an alien friend who could be treated as an alien enemy. 167 By contrast, the aliens Madison was defending had, in his word, been “invited” into the United States. 168

In sum: the modern judicial decisions limiting the term “invasion” only to attacks by an outside sovereignty are clearly erroneous and should not be followed. Rather, as the Constitution employs the words “invasion” and “invaded,” those words denote an unauthorized and uninvited intrusion of any size across a border—including significant unauthorized immigration—where the intrusion causes, or threatens to cause, detriment beyond the fact of the intrusion itself. An invasion need not be armed or even formally organized, although organization does tend to show a link between the intrusion and potential or actual detriment.

A. Insurrection

Except in cases of actual civil war, official response to insurrection is generally a matter for the police power rather than the war power. Even during civil war, the punishment of insurrectionists is likely to be handled through the civilian criminal justice system, including the prosecution of civil crimes such as treason and sedition.

To the extent permitted by a state constitution, officials may suspend the writ of habeas corpus or declare martial law, 218 so long as they do not dispense entirely with the due process guarantee of the Fourteenth Amendment. 219 If the circumstances call for it, they also may request that Congress suspend the writ. They may restrict immigration to the extent that doing so does not conflict with federal law. Obviously, they may employ other devices common in wartime, such as curfews and roadblocks.

Under the Domestic Violence Clause, the state legislature may, by due notice (“Application”) compel the federal government to suppress “domestic Violence.” 220 A state resolution to that effect probably does not need the signature of the governor, because an application to Congress is not an act of lawmaking. 221 If the state legislature cannot be convened, then the governor may issue the application. 222

B. Invasion

The Constitution’s Self-Defense Clause specifically recognizes the reserved state power to wage defensive war against invaders. 223 As documented above, 224 the Constitution’s definition of “invasion” is quite broad: It is not, as some courts have opined, 225 limited to military attack from another sovereignty. An incursion qualifies as an invasion if it is unauthorized and uninvited and causes or threatens detriment beyond the mere fact of crossing. 226

If a state is invaded, the Protection From Invasion Clause requires the federal government to protect that state. However, a state’s ability to respond to the invasion does not depend on federal compliance with the Protection From Invasion Clause. The state may react with the full panoply of measures traditionally associated with defensive war—that is, with all means necessary to repel the invasion, 227 while avoiding excessive means. 228

Thus, under the Constitution, a state facing an imminent or actual invasion may issue warnings against further invasion and erect barriers at the border. 229 It may conscript and otherwise raise troops and ships beyond its militia and National Guard establishments. 230 It may deploy those troops in all ways traditionally characteristic of defensive war, other than by issuing letters of marque and reprisal. 231 It may create internal checkpoints, fight the invaders within the state, repel them at the border, or return them whence they came. In the course of military operations, state armed forces may capture invading combatants and seize their property, or kill them if they refuse to surrender their arms. 232 The state may launch preemptive attacks and, under some circumstances, make forays into a neighboring sovereignty (including one claiming to be neutral) if that sovereignty is guilty of harboring the enemy. 233 As in cases of insurrection, the state may, consistently with its constitution, suspend the writ of habeas corpus and, of course, may ask Congress to do so as well. 234

Typically, invaders are not in allegiance to the state before the invasion. Rather, they are alien enemies or persons the state lawfully can treat as such. This renders them subject to rules different from those applied to insurrectionists. 235 Generally speaking, the state must treat captured combatants as honorable prisoners of war, unless found guilty of war crimes or qualifying as “enemies of the human race.” 236

In some cases, state policy makers may determine that international criminal organizations qualifying as hostes humani generis comprise all or part of an invasion. In cases of insurrection, a sovereign treats captives as people in allegiance who have abused their trust. In cases of invasion by alien enemies, a sovereign treats them as prisoners of war. But as for “enemies of the human race,” a sovereign may handle them either way. 237

C. May Treaties or Federal Law Impair State War Powers?

There are clear limits on the power of states to wage defensive war, even when faced with insurrection or invasion. Federal statutes or treaties may override state efforts to restrict immigration or the free flow of goods. 238 The Fourteenth Amendment prohibits dispensing with due process or equal protection of the laws, although both concepts are malleable enough to take wartime exigencies into consideration. 239 Congressional approval is necessary for mutual agreements with other states, military or otherwise, 240 although such approval can be implied. 241

More difficult is the question of the extent to which federal execution of incidental powers, such as statutes enacted under the Necessary and Proper Clause, 242 may impair further the ability of states to wage defensive war.

There are several relevant Supreme Court cases. Missouri v. Holland 243 held that when Congress legislates pursuant to a treaty, Congress is not otherwise restricted to its specifically-enumerated powers, apparently because the Necessary and Proper Clause grants Congress authority to enact laws “necessary and proper” for treaty execution. 244 In Reid v. Covert, 245the plurality opinion clarified Missouri v. Holland by stating that, although Congress may exercise otherwise-unenumerated powers when legislating pursuant to treaties, it may not adopt laws in violation of “any specific provision of the Constitution,” 246 such as the limitations in the first eight amendments of the Bill of Rights. Presumably, this would include the reservation in the Self-Defense Clause of state powers to wage defensive war.

Bond v. United States 247 qualified the rule of Missouri v. Holland further: Congressional legislation adopted pursuant to treaties should be construed when possible to avoid intruding on areas of traditional state concern.

Finally, Prinz v. United States 248 held that a law adopted for a purpose outside Congress’s specifically-enumerated powers cannot be upheld under the Necessary and Proper Clause if it intrudes on state sovereignty to such an extent that the law is not “proper.” 249 The state interest overridden in Missouri v. Holland was control over human interactions with migratory birds. The state interest protected in Prinz was freedom from federal “commandeering”—federal imposition of an administrative function on state officials. However, the power to wage defensive war is even more central to state sovereignty than the interest defended in Printz. It may be necessary to territorial integrity and, potentially, to survival. Presumably the Printz doctrine protects it against federal exercise of incidental authority.

D. Some Thoughts on Justiciability

Several Supreme Court cases have determined that the “republican Form” mandate in Article IV, Section 4 is committed to the political branches of the federal government, and, therefore, “republican Form” cases are not justiciable. 250 Without much analysis, some lower courts have extended this rule to the Protection From Invasion Clause 251 and to other aspects of reserved state territorial integrity. 252

Detailed examination of modern justiciability issues is beyond the scope of this article. Several observations may, however, assist in framing future discussion.

First: the Supreme Court’s reasons for rendering “republican Form” cases non-justiciable are based on considerations unique to that portion of Article IV, Section 4. These considerations involve matters of definition (“When is a government republican?”) and matters of practicality (“What is the retroactive and prospective legal effect of declaring a government “non-republican?”). 253 Those considerations are of limited relevance to invasion cases, because the definitional doubt is smaller, and the meaning of “invasion” can be determined by a state government having authority to do so.

Second: the courts’ opinions holding “invasion” cases to be non-justiciable also displayed the belief that the constitutional term “invasion” refers only to a military attack from a foreign government. 254 Because such an attack was not a feature of those cases, it was easier to dismiss them as non-justiciable. As demonstrated above, however, 255 that belief is clearly erroneous.

Third: The consequences from failing to enforce the insurrection and invasion mandates may be far more severe than those arising from failing to enforce the “republican Form” mandate. If Texas or Montana decided to enthrone a king, the Union could continue with all 50 states intact. Failure to protect a state against insurrection or invasion could sever or topple the Union itself. 256

Fourth: Judicial failure to enforce the federal duty to protect states from insurrection or invasion would convert a clear constitutional requirement into a mere suggestion that federal politicians could ignore at will. This, in turn, would undercut a central reason the Constitution was adopted: to “provide for the common Defence.” 257

Treating insurrection and invasion as non-justiciable has implications beyond the scope of the federal duty to protect. It also has implications for the extent of state war powers. After all, “Insurrection” and “invasion” not only trigger the federal government’s duty under the Protection From Invasion Clause, but also trigger exercise of state war powers. If the terms are too vague for courts to define for federal purposes, then they also are too vague for courts to define for state purposes. If Protection From Invasion Clause cases are held to be non-justiciable because the Constitution commits the decision of whether and how to protect states against invasion to the political branches of the federal government, then the Constitution even more clearly commits (as demonstrated by the Self-Defense Clause) the determination of whether a state has been “Invaded” or in “imminent Danger” to the state government. If redressibility issues impede justiciability in Protection From Invasion Clause cases, then they could also impede justiciability when a state has gone onto a war footing and raised an army.

To be clear: If federal officials are proceeding in good faith to crush an insurrection or repel an invasion, the courts should not second-guess their tactics. 258 But judicial intervention is appropriate when federal officials utterly neglect their duty or adopt measures so plainly insufficient as to demonstrate a lack of good faith effort.

Like the issue of justiciability, the choice of remedies against recalcitrant officials is best left to another day. We might suggest, however, that where mandamus, declaratory judgments, or injunctions are not practical, monetary damages might well be. Damages could, for example, fund or reimburse state expenses incurred in addressing the problem without federal assistance.