In March 1941, President Franklin D. Roosevelt signed the Lend-Lease Act, which authorized the president to give arms to any nation if it was in America's national interest. With that, America was able to support Great Britain without declaring war on Nazi Germany or Italy and thereby officially embroiling the country in World War II. Roosevelt convinced Congress to send aid to Great Britain on the basis that the U.S. would be defending four essential freedoms, and in August 1941, Roosevelt went so far as to secretly meet British Prime Minister Winston Churchill off the coast of Canada, after which the two issued the Atlantic Charter, a statement of Allied goals in the war. It largely reiterated the kind of ideals put forth by Woodrow Wilson a generation earlier, but it also specified that an Allied victory would not lead to territorial expansion or punitive punishment, clearly hoping to avoid what happened at Versailles at the end of World War I. Neutrality was officially over, but war was not yet on for the Americans.

Though Roosevelt could not have known it at the time, the Lend-Lease Act would quickly come to include an altogether different country than Britain. In the warm predawn darkness of June 22, 1941, 3 million men waited along a front hundreds of miles long, stretching from the Baltic coast of Poland to the Balkans. Ahead of them in the darkness lay the Soviet Union, its border guarded by millions of Red Army troops echeloned deep throughout the huge spaces of Russia. This massive gathering of Wehrmacht soldiers from Nazi Germany and its allied states - notably Hungary and Romania - stood poised to carry out Operation Barbarossa, Hitler's surprise attack against the country of his putative ally, Soviet dictator Joseph Stalin.

Operation Barbarossa was the most fateful decision of World War II, and when it gave the Soviets common cause with the British (and subsequently the Americans), the purpose of the Lend-Lease Act changed in nature as well. The bulk of Germany's formidable armed forces were committed to the offensive in the east, which relieved the pressure on the British and meant that a German attack on Britain or elsewhere in Western Europe was not going to happen, so keeping the Soviets in the war became the most essential goal of the supply program.

Getting supplies to the Soviets to help them resist the German armies became a strategic imperative, and Iran's geography of bordering the Persian Gulf to the south and Soviet territory to the north brought Iran to the front and center in the strategic supply effort. The German invasion devastated much of European Russia, but also devastated Ukraine and Belarus, other member "republics" in the Soviet Union, and the portions of the Soviet Union that bordered on Iran were Armenia, Azerbaijan and east of the Caspian, Turkmenistan.

During the fighting, the Soviet Union suffered about 25 million killed, with recent estimates placing that huge total even higher. About half of those losses were military, and half civilians. Total German military killed in action or missing in World War II were about 5.2 million, and something like 75-80% of those German military losses were on the Eastern Front. Statistics can be only approximate because of the prolonged chaotic conditions and the immense destruction. The huge area fought over was also the principal arena of the Holocaust, and a literal decimation of the population of Poland.
There were three routes available for sending aid to the Soviet Union. The first route used was the northern route for shipping, which rounded the top of Scandinavia, to the Arctic ports Murmansk and Archangel in the extreme north of the U.S.S.R. This was the shortest route, but it ran a gauntlet of German submarines, surface raiders and aircraft. Convoys assembled from ports in Britain, Canada, and Iceland, and then were shepherded by numerous naval escorts.