The Second World War posed an existential crisis for the League of Red Cross Societies. This chapter explores how the League attempted to fulfill its primary task of coordinating the work of National Societies when the movement was at war with itself, and when the Red Cross principles were flouted by the belligerent militaries and ignored by their National Societies. By focusing on the League, rather than the ICRC, the chapter broadens our understanding of the Red Cross movement during the conflict. The war illuminated some of the strengths as a network but also exposed some of its latent weaknesses. The chapter shows how, far from being a hiatus in the League’s history, the war years were critical in shaping the wider development of Red Cross humanitarianism. They accentuated the importance of American leadership for the League but also accelerated thinking over the League’s involvement in international relief efforts and sharpened its ambitions to leadership over matters relating to civilian populations in times of war. Both these factors were vital to the way the League legitimised its place within the Red Cross movement and the trans-Atlantic approach taken to articulating the role of the Red Cross in the post-war world.