States have long been considered the primary, but not only, subjects of International Law. To be considered a State, Article 1 of the Montevideo Convention (1933) sets out four criteria: a permanent population, a defined territory, a government, and the capacity to enter into relations with other states (Montevideo Convention, 1933). A State's sovereignty is here limited to its territory, over which its legal system has complete jurisdiction. However, defined territory is not uncomplicated, as States control their airspace and have a border to outer space, and coastal State’s territory encompasses maritime zones surrounding their land (Gioia, 2019). This article analyses the International and European legal framework regulating States in their maritime areas. Then, it will focus on the interaction between those legal sources and their implications for European Defence.