Key Takeaways:

• The State Practice Supplement to Limits in the Seas no. 150, released by the U.S. State Department on January 12, advances a critique of China’s maritime claims in the South China Sea that departs from Washington’s previous rationale.
• While the U.S. has historically interpreted UNCLOS as prohibiting continental states from applying straight baselines to their outlying archipelagos across the board, the State Practice Supplement instead engages Chinese maritime claims on the level of de-facto state practice and its capacity to generate customary international law which contradicts the Convention.
• The supplement relies primarily on drawing distinctions on the bases of archipelagic geography and opinio juris in an attempt to isolate Beijing’s claims as unique in their excesses and reliance on customary norms.
• This document demonstrates the Biden Administration’s willingness to bring the U.S. position on international maritime law away from a ‘purist’ reading of UNCLOS and more in line with the sensitivity to general state practice seen in the rest of the world. 
• This new formulation seems to be motivated by a desire in Washington to allow for some more pragmatically lax interpretations of UNCLOS while still maintaining the United States’ credibility as a champion of international maritime law, a useful tool in its campaign against Beijing’s ‘excessive maritime claims.’
• While the supplement is a step in the right direction, several argumentative flaws still undergird where the U.S. draws the line between ‘legitimate’ and ‘illegitimate’ baseline practice. These contradictions must be resolved if China is going to respond to U.S. criticism in good faith.