The Lend-Lease policy, formally titled An Act to Promote the Defense of the United States (Pub.L. 77–11, H.R. 1776, 55 Stat. 31, enacted March 11, 1941), was a program under which the United States supplied the United Kingdom (and British Commonwealth), Free France, the Republic of China, and later the Soviet Union and other Allied nations with food, oil, and materiel between 1941 and 1945. Loaned on the basis that such help was essential for the defense of America, this aid included warships and warplanes, along with other weaponry. It was signed into law on March 11, 1941, and ended on September 20, 1945. In general, the aid was free, although some hardware (such as ships) were returned after the war. In return, the U.S. was given leases on army and naval bases in Allied territory during the war. Canada operated a similar smaller program called Mutual Aid.

A total of $50.1 billion (equivalent to $690 billion in 2020) worth of supplies was shipped, or 17% of the total war expenditures of the U.S. In all, $31.4 billion went to the United Kingdom, $11.3 billion to the Soviet Union, $3.2 billion to France, $1.6 billion to China, and the remaining $2.6 billion to the other Allies. Reverse Lend-Lease policies comprised services such as rent on air bases that went to the U.S., and totaled $7.8 billion; of this, $6.8 billion came from the British and the Commonwealth. The terms of the agreement provided that the materiel was to be used until returned or destroyed. In practice, very little equipment was returned and most was destroyed during the war. Supplies that arrived after the termination date were sold to the United Kingdom at a large discount for £1.075 billion, using long-term loans from the United States. Canada's Mutual Aid program sent a loan of $1 billion and $3.4 billion in supplies and services to the United Kingdom and other Allies.Lend-Lease effectively ended the United States' pretense of neutrality which had been enshrined in the Neutrality Acts of the 1930s. It was a decisive step away from non-interventionist policy and toward open support for the Allies. Roosevelt's top foreign policy advisor Harry Hopkins had effective control over Lend-Lease, making sure it was in alignment with Roosevelt's foreign policy goals.