The Lend-Lease policy, formally titled An Act to Further Promote the Defense of the United States, (enacted March 11, 1941) was a program under which the United States supplied Great Britain, the USSR, Free France, the Republic of China, and other Allied nations with materiel between 1941 and August 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 and nine months before the U.S. entered the war in December 1941.
This program effectively ended the United States' pretense of neutrality and was a decisive step away from non-interventionist policy, which had dominated United States foreign relations since 1931.
Lend-Lease would help the British and Allied forces win the battles of future years; the help it gave in the battles of 1941 was trivial. In 1943–1944, about a quarter of all British munitions came through Lend-Lease. Aircraft (in particular transport aircraft) comprised about a quarter of the shipments to Britain, followed by food, land vehicles and ships.
Even after the United States forces in Europe and the Pacific began to reach full strength in 1943–1944, Lend-Lease continued. Most remaining allies were largely self-sufficient in front line equipment (such as tanks and fighter aircraft) by this stage, but Lend-Lease provided a useful supplement in this category even so, and Lend-Lease logistical supplies (including motor vehicles and railroad equipment) were of enormous assistance.
The USSR was highly dependent on rail transportation, but the war practically shut down rail equipment production: only about 92 locomotives were produced. 2,000 locomotives and 11,000 railcars were supplied under Lend-Lease. Likewise, the Soviet air force received 18,700 aircraft, which amounted to about 14% of Soviet aircraft production (19% for military aircraft).