During the Great Depression of the 1930s, Roosevelt created the New Deal to provide relief for the unemployed, recovery of the economy, and reform of the economic system. His most famous legacies include the Social Security system and the regulation of Wall Street. His aggressive use of an active federal government reenergized the Democratic party. Roosevelt built the New Deal coalition that dominated politics into the 1960s. He and his wife Eleanor Roosevelt remain touchstones for American liberalism. The conservatives fought back, but Roosevelt consistently prevailed until he tried to pack the Supreme Court in 1937, and the Conservative coalition formed to stop New Deal expansion.

After 1938, Roosevelt championed rearmament and led the nation away from isolationism as the world headed into World War II. He provided extensive support to Winston Churchill and the British war effort before the attack on Pearl Harbor pulled the U.S. into the fighting. During the war, Roosevelt and the United States provided decisive leadership against Nazi Germany and made the United States the principal arms supplier and financier of the Allies who defeated Germany, Italy and Japan. Roosevelt led the United States as it became the Arsenal of Democracy, putting 16 million American men and women into uniform.

On the homefront his term saw the end of unemployment, restoration of prosperity, significant new taxes and controls, 120,000 Japanese and Japanese Americans sent to relocation camps, and new opportunities opened for African Americans and women. As the Allies neared victory, Roosevelt played a critical role in shaping the post-war world, particularly through the Yalta Conference and the creation of the United Nations. Roosevelt died on the eve of victory in World War II and was succeeded by Vice President Harry S. Truman.