Over 50,000 international treaties are in force today, covering nearly every aspect of international affairs and nearly every facet of state authority. And yet many observers continue to argue that international law-with its general absence of central enforcement and its typically voluntary character-is ineffective. This Article assesses and responds to this challenge. Building upon insights from both political science and legal scholarship, it offers a theory of state decisions regarding treaty law that accounts for the key ways in which such law shapes state behavior. This integrated theory of international law seeks to explain why countries would commit to treaties that potentially constrain their behavior and how treaties, once accepted...