Despite strong neutralist sentiments among members of Congress and the general public, Roosevelt recognized that U.S. national security depended on Great Britain's survival. In March 1941, he introduced the Lend-Lease Act in order to supply Britain and her Allies with war material. Roosevelt oversaw the development of military strategy and often conferred with British prime minister Winston Churchill. Roosevelt and Churchill met with Soviet leader Joseph Stalin at Teheran in 1943 and Yalta in 1945. At these meetings, the leaders of the three principal allied nations not only discussed wartime strategy, but also planned for the postwar order.  Many historians have criticized Roosevelt for being too trusting of Stalin, who established Communist puppet states in Eastern Europe after the war. 

Roosevelt died on April 12th 1945, shortly before the end of the war, after he collapsed from a cerebral hemorrhage.  The same day in Washington, Harry Truman was sworn in as president. 

During his term as Vice-President, Truman was rarely consulted by Roosevelt.  However, after he succeeded Roosevelt to the Presidency, he adapted quickly to the role and the heavy responsibility.  To avoid a bloody invasion of Japan and to "scare" the Soviets, Truman was the one who made the decision to use the atomic bomb on Japan, which subsequently ended war with Japan and started another war: the Cold War.