After winning reelection, FDR felt confident in stepping up American aid to the Allies. He pushed for passage of the Lend-Lease Act in March 1941, which allowed the president to lend or lease supplies to any nation deemed “vital to the defense of the United States,” such as Britain. FDR extended lend-lease aid to the Soviet Union after Germany invaded in November 1941. The U.S. also helped the Allies by tracking German submarines and warning the British of their location, and by convoying British ships carrying lend-lease supplies—that is, surrounding British ships with U.S. Navy vessels ordered to attack any menacing vessel.

In August 1941, FDR met with British Prime Minister Winston Churchill on a British ship off Newfoundland. The two discussed military strategy and issued the Atlantic Charter, which outlined their ideal postwar world: among other provisions, it called for disarmament and freedom of the seas. In response to a German attack against a U.S. destroyer, Roosevelt issued the shoot-on-sight order in September 1941, which authorized American naval patrols to fire on all Axis ships found between the U.S. and Iceland. After American destroyers were twice attacked in October, Congress authorized the arming of merchant ships.

The final provocation for American entry into the war came from Japan, which had joined the Rome-Berlin-Tokyo Axis in September 1940 by signing the Tripartite Pact. Japan’s desires to build an East Asian empire had alarmed the U.S. since Japan’s invasion of China in 1931. In September 1940, Japanese forces continued their invasion into French Indochina. The U.S. responded to this invasion as it had to other invasions in the past: it added items to a lengthy list of embargoed Japanese goods, and eventually froze all trade with Japan. In 1941, U.S. intelligence became aware of plans for a Japanese attack and sent out warnings to commanders of U.S. bases in the Pacific, but most American officials did not believe that the threat was immediate. These officials were proved wrong on December 7, 1941, when, in an attempt to destroy American sea power in the Pacific, Japanese planes bombed the U.S. base at Pearl Harbor. The Japanese destroyed nearly 200 aircraft, eight battleships, three cruisers, and three destroyers. Almost 2,400 Americans died. On December 8, the Senate voted unanimously in favor of FDR’s request for a declaration of war on Japan. The House passed the declaration over only one dissenting vote. On December 11, Germany and Italy joined Japan in war against the U.S.

On January 1, 1942, representatives of 26 nations signed the Declaration of the United Nations, pledging support for the Atlantic Charter and vowing not to make separate peace agreements with the Axis powers.

Although the U.S. had begun preparing for war during the summer of 1940, war production comprised only 15 percent of the nation’s industrial output in 1941, and U.S. armed forces were seriously understaffed and undersupplied. During the next four years, war production on the home front churned into high gear.

In 1942, FDR created the Joint Chiefs of Staff to oversee America’s rapidly expanding military. By the end of the war, more than 15 million men had served in the U.S. armed forces. About 350,000 women served as well, most in the Women’s Army Corps (WAC) and Women Accepted for Voluntary Emergency Service in the navy (WAVES). The air force, a minor corps at the outset of the war, grew substantially and gained a measure of autonomy during the war. The Joint Chiefs also established the Office of Strategic Services (OSS) to assess the enemy’s assets and liabilities, conduct espionage, and gather information to be used in strategic planning.

The U.S. mobilized industry to assist in the war effort. The War Production Board, created in 1942, oversaw the production of thousands of planes, tanks, and artillery pieces required for the war. The War Production Board allocated scarce resources and offered incentives for civilian firms to produce military goods. The last civilian car was produced in the U.S. in 1942, after which plants were redesigned to produce tanks, planes, weapons, and munitions. By the end of the war, the U.S. had built about 300,000 aircraft, 85,000 tanks, 375,000 artillery pieces, 2.5 million machine guns, and 90,000 sea ships—more war material than the four Axis powers, combined, had produced. This feat was accomplished through substantial investment of capital and the development of new, highly efficient manufacturing techniques.

The federal budget multiplied tenfold between 1940 and 1945. U.S. expenditures during World War II totaled nearly twice the amount spent by the U.S. government in its previous 150 years of existence. Spending on war production precipitated a shift in American income distribution, with the share of national income allocated to the richest Americans decreasing and that allocated to the middle class doubling. The Revenue Act of 1942, passed to help pay for the war, increased taxes for the wealthiest Americans.

War spending, accompanied by the draft, ended the high rate of unemployment which had not rebounded from from the Great Depression. Organized labor grew strong and wealthy during World War II, with union membership growing by about 60 percent. Although most unions abided by a no-strike policy, unions secured new benefits (such as fewer hours and better health plans) for their members, partially as a concession from the National War Labor Board, which limited wage increases to avoid inflation. Union power suffered a setback when a series of coal miners’ strikes provoked Congress to pass the Smith-Connolly War Labor Disputes Act in June 1943, which limited the right to strike in key industries and allowed the president to take control of any firm beset by strikes.

The Office of Price Administration waged a battle against inflation and the over-use of resources. The OPA oversaw a rationing program designed to curb new purchases and conserve materials, in particular gas, sugar, coffee, butter, and meat. The American people largely complied with these efforts by forsaking many goods—for example, implementing “meatless Tuesdays”—and by planting “victory gardens” and conducting collection drives to gather materials for recycling. Another tactic aimed at financing the war was the sale of war bonds.

FDR relied on the Office of Censorship and the Office of War Information to regulate the communications of American citizens. The former, created shortly after the U.S. entry into the war, examined all letters sent overseas and worked with media firms to control information broadcast to the people. The latter, formed in June 1942, employed artists, writers, and advertisers to shape public opinion by explaining the reasons for U.S. entry in the war and by portraying the enemy as barbaric and cruel.

War advertising encouraged women to actively participate in the war effort. During World War II, even more so than in World War I, women entered the workforce to fill the vacancies left by men at war and aid in the production of war materials. The image of Rosie the Riveter, a muscle-bound woman with a rivet gun, was pervasive and effective. Increasing numbers of women entered the workforce to perform all sorts of jobs. The roles women played in the armed forces helped create new respect for the working woman. Women, however, were paid far less than working men, and traditional notions of gender roles prevailed throughout the war.

During World War II, the U.S. government rounded up more than 110,000 Japanese immigrants and U.S.-born Japanese-Americans and sent them to relocation centers guarded by military police. Military leaders, West Coast farmers, and others rationalized this policy as necessary to prevent acts of sabotage and espionage in support of Japan. In 1942, FDR authorized this relocation in Executive Order 9066, and in 1944 the Supreme Court upheld the constitutionality of the order in Korematsu v. U.S. In 1988, Congress voted to pay reparations of $20,000 to every internee still living.

FDR ran for reelection once again in 1944, in the midst of World War II, with moderate Democrat Harry S. Truman as his running mate. FDR won an unprecedented fourth term, though by his narrowest margin ever. Shortly after Roosevelt’s fourth term began, he died of a cerebral hemorrhage in April 1945, leaving Truman to oversee the war effort.