The Lend-Lease Act was a pivotal piece of legislation passed by the United States in March 1941 that allowed the country to provide military aid to foreign nations during World War II. This act marked a significant shift in U.S. foreign policy from isolationism to active involvement, as it enabled the transfer of arms, equipment, and supplies to allies like Britain, China, and later the Soviet Union without immediate payment. By supporting its allies, the U.S. aimed to strengthen their ability to fight against Axis powers while also protecting its own interests.