Lend-Lease
The 1941 program that let America arm the Allies without joining the war, and quietly bought the dismantling of the British Empire.
Lend-Lease was the legal device that let the United States supply the countries fighting Hitler while remaining formally neutral. Signed into law in March 1941, it authorized the president to lend, lease, or otherwise transfer war material to any country whose defense he deemed vital to American security, with payment deferred, waived, or taken in kind. Over the following four years it moved more than fifty billion dollars of ships, aircraft, trucks, food, and raw material, and it is the single clearest demonstration of what industrial capacity is worth in a world war.
The problem it solved
By late 1940 Britain was fighting alone and running out of money. American neutrality law required cash and carry: belligerents could buy US goods only if they paid up front and shipped them themselves. Britain's dollar reserves were nearly gone, and Churchill wrote to Roosevelt that the moment was approaching when Britain could no longer pay.
Roosevelt's answer was a metaphor rather than an argument. If your neighbor's house is on fire, he told a press conference, you lend him your garden hose. You do not haggle over the price of the hose first; you get it back, or he replaces it, when the fire is out. In a fireside chat he called for America to become the great arsenal of democracy. The isolationist opposition understood perfectly well that a hose lent to a burning house tends to be followed by the man holding it, and fought the bill hard. It passed anyway.
What flowed
The scale is the point. American factories supplied Britain, the Soviet Union, China, Free France, and dozens of smaller recipients. Britain and the Commonwealth took the largest share, the Soviet Union the second largest.
For the Soviet Union the composition mattered more than the total. The Red Army built its own tanks and artillery superbly and in enormous quantity, but it was chronically short of the unglamorous things that turn an army into a mobile one. Lend-Lease supplied hundreds of thousands of trucks and jeeps, huge quantities of aviation fuel, explosives, telephone cable, railway locomotives and rolling stock, and vast tonnages of food. Soviet historiography spent the Cold War minimizing this and post-Soviet historians have been franker: Zhukov privately acknowledged that the Soviet Union could not have continued the war without American explosives, steel, and trucks. The deep operations of 1944 that carried the Red Army to Berlin were driven, quite literally, on American wheels.
Delivery was its own war. The Arctic convoys to Murmansk ran through U-boat packs and German aircraft off Norway. The Persian Corridor ran supplies up through occupied Iran, which is a large part of why Britain and the Soviet Union invaded Iran in 1941 and deposed its shah.
The invisible price
What makes Lend-Lease more than a logistics story is what Washington charged for it, which was not money.
The consideration Britain gave was policy. Successive agreements committed Britain to move toward eliminating discrimination in international commerce, which meant dismantling the imperial preference system that gave the empire's trade to itself. American negotiators returned to this point repeatedly, and Keynes, negotiating for Britain, understood exactly what was happening: the price of survival was the economic architecture of the empire. The Atlantic Charter's clause on self-determination pointed the same way, and Churchill's insistence that it did not apply to the British Empire persuaded nobody in Washington.
When the program was abruptly terminated in August 1945, days after the Japanese surrender, Britain was left without dollars and without the trading system it had relied on. It negotiated a separate postwar loan on hard terms, and the final payment on that loan was made in December 2006.
That is the honest summary. Lend-Lease was an act of extraordinary strategic generosity and a transfer of power. It kept Britain and the Soviet Union in the war, and it moved the center of the world economy across the Atlantic without a shot being fired at the country receiving it.