Abstract
The Gulf of Tonkin is a shared bay between Vietnam and China, which was delimited by the Agreement between the People’s Republic of China and the Socialist Republic of Viet Nam on the delimitation of the territorial seas, exclusive economic zones and continental shelves of the two countries in Beibu Gulf (the Chinese name of the Tonkin Gulf)/Bac Bo Gulf on 25 December 2000. The determination of internal waters, territorial seas, and related maritime zones depends on the declaration of baselines in the Gulf of Tonkin. After more than 20 years, China (March 2024) and Vietnam (February 2025) have issued significant unilateral declarations regarding baselines. These developments raise concerns about the impact of such declarations on the 2000 Delimitation Agreement, Vietnam-China relations, and the consistency of the 1982 United Nations Convention on the Law of the Sea (UNCLOS). This article will analyze three main aspects: (i) the baselines of China and Vietnam in the Gulf of Tonkin, (ii) the establishment of baselines in light of UNCLOS, (iii) the impact of the new baseline systems on the 2000 Delimitation Agreement and Vietnam-China cooperation.