For much of human history, water was a standard weapon of war. In the post–World War II period, however, nation-states in international conflict have made concerted efforts to restrain the weaponization of water. Distinct from realist and rationalist explanations, the historical record reveals that water has come to be governed by a set of intersubjective standards of behavior that denounce water's involvement in conflict as morally taboo. How did this water taboo develop, and how does it matter for nation-states? Focused process-tracing illuminates the taboo's development from the 1950s to the 2010s, and indicates that (1) a moral aversion to using water as a weapon exists; (2) this aversion developed through cumulative mechanisms of taboo evolution over the past seventy years; and (3) the taboo influences states at both an instrumental level of compliance, and, in recent decades, a more internalized level. These findings offer new avenues for research and policy to better understand and uphold this taboo into the future.