This chapter analyses the invention and development of ‘operational law’ by US military lawyers and commanders in the decades following the Vietnam War. Operational law is a pragmatic, practitioner-oriented, and military-friendly framework for interpreting rights and responsibilities under international, domestic and military law. This chapter shows how operational law helped the US military to put these legal regimes to work in the service of achieving military objectives, as military lawyers helped create a broadly permissive albeit bounded legal environment for military operations. The birth of operational law laid the foundations for a new symbiosis between commanders and lawyers, providing them with a common vocabulary and a shared understanding of the mission—factors which were later crucial to the integration of military lawyers into targeting operations.