What is the role of rhetoric and argumentation in international relations? Some argue that it is little more than ‘cheap talk’, while others say that it may play a role in persuasion or coordination. However, why states deploy certain arguments, and why these arguments succeed or fail, is less well understood. I argue that, in international negotiations, certain types of legal frames are particularly useful for creating winning arguments. When a state bases its arguments on constitutive legal claims, opponents are more likely to become trapped by the law: unable to develop sustainable rebuttals or advance their preferred policy. To evaluate this theory, I apply qualitative discourse analysis to the US arguments on the crime of aggression at the Kampala Review Conference of the International Criminal Court – where the US advanced numerous arguments intended to reshape the crime to align with US interests. The analysis supports the theoretical propositions – arguments framed on codified legal grounds had greater success, while arguments framed on more political grounds were less sustainable, failing to achieve the desired outcomes. These findings further develop our understanding of the use of international law in rhetoric, argumentation, and negotiation.