This chapter offers a synthetic overview of the range of international law issues that arose during the course of the Vietnam War, especially as Americans took over from the French after Dien Bien Phu in 1954 and moved towards massive escalation between 1964 and 1973. The chapter begins with the debate about what law applied to the conflict, which turned on the legal status of South Vietnam. The chapter then asks what claims were possible and plausible when it came to the legality of American intervention in the war. Next, the chapter addresses the different kinds of warfare in which the United States engaged, from its bombing campaigns over North Vietnamese territory and waters to the changing forms of its counterinsurgency in the South and, later, across the Cambodian border. Finally, the chapter concludes by examining the legal legacy of Vietnam: not only how it led to the most significant substantive development of the laws of war since the Geneva Conventions, the First and Second Additional Protocols, but also, and equally importantly, how it ensured that international law would play (for good or ill) a central role in debate over and analysis of all future conflicts.