This chapter examines one of the most contentious issues in the jus ad bellum: whether and when international law permits a state to use force unilaterally to rescue its nationals abroad when their lives or security are threatened. It first considers the definition of the phrase ‘rescuing nationals abroad’ and the legal scope and legal nature of the justification based on the necessity of carrying out such an act. It analyses the opinion of the International Court of Justice concerning the matter before assessing the current position of international law on the permissibility of rescuing nationals abroad. It also discusses whether the use of force to rescue nationals abroad can be invoked for humanitarian assistance purposes involving non-nationals. The chapter shows that the notion of ‘rescuing nationals abroad’ is ambiguous from a legal perspective and that the legality of using force to rescue nationals abroad has remained unclear since 1945.