As a leading figure of "War Progressives", Roosevelt was a vocal supporter for the United States' entry on World War II at the end of 1930s, which making him at odd with incumbent President Hiram Johnson, a fellow Progressive who belong to the party's isolationist wing. With his intense campaign to curb the isolationist sentiments among American populace by declaring his goal is to make America the "Arsenal of Democracy", which would supply munitions to the Allies, Roosevelt and his supporters dominated the 1940 Progressive National Convention, making the party officially supported the participation on the war.
Roosevelt was elected as the 30th President after won the 1940 election against against Republican Wendell Willkie and Liberal Cordel Hull with 55% of the popular vote and 30 of the 38 states, and thus winning almost 70% of the electoral vote (302 of 433 total votes). By June 1941, Roosevelt began to expand and re-equip the Army and Navy in preparation for possible war. After joint Italian-Spanish naval attack on Havana Harbor on December 20, 1941, he made war on Spain, Italy, and Germany. During the war, he worked closely with British Prime Minister Winston Churchill, Soviet leader Sergei Kirov, and Japanese President Nagayama Yoshida in leading the Allies against Nazi Germany, Fascist Italy, Falangist Spain, and Militarist China in World War II. He also supervised the mobilization of the U.S. economy to support the war effort.
As an active military leader, Roosevelt implemented a war strategy on two fronts that ended in the defeat of the Axis Powers and the development of the world's first nuclear bomb. His work also influenced the later creation of the United Nations and Bretton Woods. During the war, unemployment dropped to 2%, relief programs largely ended, and the industrial economy grew rapidly to new heights as millions of people moved to wartime factory jobs or entered military service. Roosevelt's health was seriously worsened during his administration, and he died of heart attack almost two weeks into his second term.