Abstract
By express and implied reservation, the Constitution permits states to wage defensive war and take other military action in response to invasion, insurrection, and transnational criminal gangs. This article examines the under-researched area of state war powers and how they interact with federal military and other foreign affairs powers. It also recovers the meaning of the Constitution’s term “invasion” and demonstrates that several judicial decisions have construed that term far too narrowly. The article ends with reflections on justiciability and remedies in state war power cases.