Theoretical debates over international legal regimes, such as the United Nations Convention on the Law of the Sea (UNCLOS), have tended to revolve around the constraints international law may or may not place on confrontational state behaviour, leaving its constitutive aspects underexplored. This talk offers a counterintuitive explanation for why tensions in the South China Sea have risen, not declined, in the UNCLOS era. The new international regime reconstituted China and its neighbours’ interests in jurisdiction at sea to produce harder, yet also more ambiguous claim. Tracing four representative cases of China’s new and assertive patterns of behaviour in the South China Sea in 2007-2008, it shows that, intertwined with rising material capabilities and resource insecurity, the new challenges and opportunities presented by the implementation of the legal regime were crucial drivers of Beijing’s policy shift on its maritime periphery. Using PRC maritime law enforcement agency materials, internal government advisory papers, State Department cables, official statements and research interviews, the paper identifies three causal pathways linking the UNCLOS to China’s altered behaviour. International law not only constrains confrontational state actions, but can also authorise, enable, and catalyse them.