Lend-Lease (Public Law 77-11) was the program under which the United States of America supplied the United Kingdom, the Soviet Union, China, Free France, and other Allied nations with materiel between 1941 and 1945. It was signed into law on March 11, 1941, a year and a half after the outbreak of World War II in Europe in September 1939 but nine months before the U.S. entered the war in December 1941. Formally titled An Act to Further Promote the Defense of the United States, the Act effectively ended the United States' pretense of neutrality.

A total of $50.1 billion (equivalent to $647 billion today) worth of supplies were shipped: $31.4 billion to Britain, $11.3 billion to the Soviet Union, $3.2 billion to France, and $1.6 billion to China. Reverse Lend-Lease 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 were to be used until time for their return or destruction. Supplies after the termination date were sold to Britain at a discount for £1.075 billion using long-term loans from the United States. Canada operated a similar program that sent $4.7 billion in supplies to the United Kingdom and the Soviet Union. The United States did not charge for aid supplied under this legislation.