The Lend-lease Act of March 11, 1941 permitted the President of the United States to "sell, transfer title to, exchange, lease, lend, or otherwise dispose of, to any such government [whose defense the President deems vital to the defense of the United States] any defense article". It thus extended Cash and Carry and modified the sense of neutrality. The value of the items to be lent were not to exceed $1,300,000,000 in total. US President Franklin Delano Roosevelt approved US$1 billion in Lend-lease aid to Britain on October 30, 1941. Britain was still repaying this into the next century.

The act is generally known as "lend-lease" in the US but "lease-lend" in the UK. In fact neither term appears in the true title of the act, which is "Further to promote the defense of the United States, and for other purposes.". See [1] for the text of the act. More information, and images of the text, from [2].

Earlier, the 1940 Destroyers for Bases Agreement had seen fifty obsolete destroyers transferred to the Royal Navy and the Royal Canadian Navy in exchange for base rights in the Caribbean.

Lend-lease was a critical factor in the eventual success of the Allies in World War II, particularly in the early years when the United States was not directly involved and the entire burden of the fighting fell on other nations, notably those of the Commonwealth. Although Pearl Harbor, and a German Declaration of War, brought the US into the war in December 1941, the task of recruiting, training, and equipping US forces, and then transporting them to the war zones could not be completed overnight: through 1942, and to a lesser extent 1943, the other Allies continued to be responsible for most of the fighting, and the supply of military equipment under lend-lease was a significant part of their success.

Even after the United States forces in Europe and the Pacific began to reach full-strength in 1943-1944, lend-lease continued. Most remaining belligerents were largely self-sufficient in front-line equipment (such as tanks, escort aircraft carriers and fighter aircraft) by this stage, but lend-lease provided a useful supplement in this category even so, and lend-lease logistical supplies (including trucks, jeeps, landing craft, and above all the Douglas DC-3 transport aircraft) were of enormous assistance.