US President Franklin D. Roosevelt recognized the global threat from National Socialist Germany and Imperial Japan long before most of America. Despite strong bipartisan isolationism in Congress (not to mention German –American was the largest ethnicity in America at the time), FDR managed to conclude the Destroyers for Bases Agreement on 2 Sept 1940 in which 50 near obsolete “four stacker” Wilkes and Clemson class destroyers were given to Great Britain for 99 year leases of British bases in the Caribbean and New Foundland. After securing victory in the 1940 election, FDR announced that the US would become the “Arsenal of Democracy” on 2 December 1940 with the knowledge that numerous secret State and War Department studies concluded that Great Britain after the Fall of France was “doomed without full economic and financial support from the United States”.
That winter, stories from the Luftwaffe Blitz and U boats in the North Atlantic shifted public opinion in the US. On 11 March 1941, Congress passed the Lend Lease Act and FDR signed it into law the same day. The Lend Lease Act provided military supplies to Great Britain, Free France, and other countries fighting Germany without “cash up front” for additional leases on Allied bases, most notably Danish Iceland. This was in direct contradiction to the Neutrality Acts which dominated US foreign policy for over a decade.
The Lend Lease Act would eventually provide over a quarter of Great Britain’s and the Commonwealth’s military supplies. In the short term it provided Canada with the ability to take over the convoy escort duties in the Western Atlantic (Canada by the end of the war would have the 3rd largest navy in the world after the US and GB). Additionally, the US would provide escorts for convoys in American territorial waters, an area that FDR would continue to extend farther and farther away from any American coast. Lend Lease dropped any pretense that America was going to stay neutral in the war in Europe.