Since the victory of Mao Zedong’s Communist forces in 1949, the People’s Republic of China (PRC) has laid claim to Taiwan. In 2005, the PRC adopted a law stating that China can use force against Taiwan, officially known as the Republic of China, if it undertakes to form an independent state. This law is an expression of the One-China policy: the idea that mainland China and Taiwan are part of the same country. However, present-day Taiwan is increasingly described as a de facto state with its own people, territory, government, and capacity for international relations. This Note asks whether international law on the use of force protects Taiwan from attack by China, given that Taiwan has many characteristics of a state but has not been formally recognized as such. Part I of the Note summarizes the debate over Taiwan’s statehood. Part II lays out the argument that non-state entities have no protection under international law on the use of force. This argument relies on a Westphalian conception of the international system, positing that states are the only subjects of international law. The Note then poses three “post-Westphalian” challenges to that argument: first, that “peoples” in pursuit of self-determination have legal protection from attack by states; second, that the United Nations Charter has been interpreted to forbid changing non-state entities’ legal status by force; and third, that states have an obligation under Article 33 to resolve their disputes without threatening international peace and security. Part III applies this legal framework to Taiwan. It finds that though the two sides of the debate are incommensurable because they are based on different understandings of international law, Taiwan’s geopolitical situation shows that arguments based on the Westphalian conception of statehood create absurd results. The post-Westphalian view that allows Taiwan limited rights under international law on the use of force better comprehends the geopolitical reality of contested states.