Do domestic legal systems affect states' propensity to form military alliances? This article, building upon the existing research in international relations, adopts a socio-legal approach to understanding international treaty making. By focusing on the essence of international negotiations—communication between states' representatives—I argue that negotiating parties who share a common legal language have a common a priori understanding concerning concepts under discussion. Domestic laws operating within states impact the process of creation of international law embodied in treaties. Empirical analyses show that states with similar legal systems are more likely to form military alliances with one another. Additionally, domestic legal systems influence the way that states design their alliance commitments. In general, my findings suggest that the influence of domestic laws does not stop at “the water's edge.” It permeates the interstate borders and impacts the relations between states, especially the treaty negotiating and drafting process. International negotiators bring their legal backgrounds to the negotiating table, which influences both their willingness to sign treaties and the design of the resulting agreements.