World War II: United States President Franklin D. Roosevelt signs the Lend-Lease Act into law, allowing American-built war supplies to be shipped to the Allies on loan.

World War II or the Second World War was a global conflict between two coalitions: the Allies and the Axis powers. Nearly all of the world's countries participated, with many nations mobilising all resources in pursuit of total war. Tanks and aircraft played major roles, enabling the strategic bombing of cities and delivery of the first and only nuclear weapons ever used in war. World War II is the deadliest conflict in history, causing the death of 70 to 85 million people, more than half of whom were civilians. Millions died in genocides, including the Holocaust, and by massacres, starvation, and disease. After the Allied victory, Germany, Austria, Japan, and Korea were occupied, and German and Japanese leaders were tried for war crimes.

Franklin Delano Roosevelt, also known as FDR, was the 32nd president of the United States from 1933 until his death in 1945. He is the longest-serving U.S. president, and the only one to have served more than two terms. His first two terms were centered on combating the Great Depression, while his third and fourth saw him shift his focus to America's involvement in World War II.

Lend-Lease, formally the Lend-Lease Act and introduced as An Act to Promote the Defense of the United States, was a policy under which the United States supplied the United Kingdom, the Soviet Union, France, the Republic of China, and other Allied nations of the Second World War with food, oil, and materiel between 1941 and 1945. The aid was given free of charge on the basis that such help was essential for the defense of the United States.

The Allies, formally referred to as the United Nations from 1942, were an international military coalition formed during World War II (1939–1945) to oppose the Axis powers. Its principal members were the "Big Four" – the United Kingdom, United States, Soviet Union, and China.