The 1941 US program that provided aid to Allied nations before American entry into World War II was Lend-Lease.
President Franklin D. Roosevelt signed the Lend-Lease Act on 11 March 1941, allowing the United States to transfer weapons, food, oil, vehicles, and other supplies to countries whose defense was considered vital to US security. Britain was the principal early recipient, receiving material while fighting Germany before the United States entered the war.
The program also aided the Soviet Union after Germany invaded in June 1941, as well as China, France, and other Allied governments. It replaced the earlier Cash and Carry approach, which required belligerents to pay immediately and transport purchases themselves. Lend-Lease made it possible to extend support without treating every shipment as an ordinary commercial sale.
The Truman Doctrine belongs to the Cold War, while the Marshall Plan funded European reconstruction after World War II. Lend-Lease did not mean the United States formally joined the war in 1941; American forces entered after Japan attacked Pearl Harbor on 7 December 1941.