America was not in a position do do anything when WWII started, she had a military that was outdated in terms of operations and personel, the model was based upon WWI operations, the personal were conscripts.
George Marshall changed all of that and was instrumental in Lend Lease, a great read is
Winning the Peace: The Marshall Plan and America's Coming of Age as a Superpower by Nicolaus Mills.
Also lets remember that there were the Baltic conveys that supplied aid and munitions to the Soviets.
Arctic convoys of World War II - Wikipedia, the free encyclopedia