The first of two principles contained in the "Just War Theory," Jus Ad Bellum and Jus In Bello. Jus Ad Bellum in Latin means "The Law to War." It defines standards by which a country can declare war and maintain that their reasons were "just." The term was coined in the same era in which the League of Nations originated. However, it was not used in doctrine until the late 1940s. Its principles include a declaration of war must have a just cause, proper authority, right intention, resonable probability of success, and whose ends are proportional to the means.
Under the guildlines of Jus Ad Bellum, France and England had proper cause to declare war on Nazi Germany, who was the obvious aggresser in the second world war.