The making of international law through multilateral conventions and adjudication often leads to periods of legal uncertainty, times in which there are alternative rules and divergent views on how they ought to be applied to particular cases. I argue that legal uncertainty gives states opportunities and incentives to formulate excessive unilateral claims, thus making disputes more likely to arise. I illustrate my argument with a comprehensive analysis of maritime boundary disputes in the aftermath of the Second World War. In this period, the law regulating maritime boundary-making has been marked by varying degrees of uncertainty due to different rules and interpretations proffered by various multilateral and judicial lawmaking attempts. I find strong evidence that high legal uncertainty is associated with an increased probability of dispute onset. The analysis calls for an important rethinking of the impact of legalization on international affairs, both in maritime boundary-making and in other issues areas.