As president of the United States amid global depression and global war, Franklin Roosevelt encountered several significant diplomatic challenges, most notably in reacting to international economic disorder, realigning traditional US policies inside and outside the Western Hemisphere, interacting with aggressor powers in Europe and Asia, and eventually cooperating in the Grand Alliance to resist aggression in the Second World War. Frequently working through intermediaries and seeking to establish personal relationships to achieve his strategic priorities, Roosevelt pragmatically focused on winning the war once the United States became a full participant. He also laid the foundations for the postwar international system. Moreover, his calibrated implementation of Wilsonian ideals in a more pragmatically oriented multilateral cooperation still shapes current debates about American foreign policy in a new age when national security threats emanate from non‐state actors.