This supply route originated in the US and UK with ships sailing around the Cape of Good Hope to the Persian Gulf. From there, the materiel transited Iran to the USSR. Other supply routes included the Northern route across the Arctic, and the Pacific route which handled US cargo at Vladivostok and then used the Trans-Siberian Railway across the USSR.
This Persian Route became the only viable, all-weather route to be developed to supply Soviet needs.
English-language official documents from the Persian Corridor period continue to make the word "Persia" interchangeable with the name of Iran. In correspondence by the government of the United Kingdom, usage of "Persia" over "Iran" was chosen by Winston Churchill to avoid possible confusion with neighbouring Iraq.
Overthrow of the Shah
Following Germany's invasion of the USSR in June 1941, Britain and the Soviet Union became allies. Britain and the USSR saw the newly opened Trans-Iranian Railway as an attractive route to transport supplies from the Persian Gulf to the Soviet Union. Britain and the USSR used concessions extracted in previous interventions to pressure Iran (and, in Britain's case, Iraq) into allowing the use of their territory for military and logistical purposes. Increased tensions with Britain led to pro-German rallies in Tehran. In August 1941, because Reza Shah refused to expel all German nationals and come down clearly on the Allied side, Britain and the USSR invaded Iran, arrested the monarch and sent him into exile to South Africa, taking control of Iran's communications and the coveted railway.
In 1942 the United States, now an ally of Britain and the USSR in World War II, sent a military force to Iran to help maintain and operate sections of the railway. The British and Soviet authorities allowed Reza Shah's system of government to collapse, and they limited the constitutional government interfaces. They installed Reza Shah's son, Mohammad Reza Pahlavi onto the Iranian/Persian throne.
The new Shah soon signed an agreement pledging full non-military logistical cooperation with the British and Soviets, in exchange for full recognition of his country's independence, and also a promise to withdraw from Iran within six months of the war's conclusion (these assurances later proved essential in securing his country's independence after the war). In September 1943, the Shah went further, and he declared war on Germany. He signed the Declaration by United Nations entitling his country to a seat in the original United Nations. Two months later, he hosted the Tehran Conference between Churchill, Roosevelt, and Stalin.
The presence of so many foreign troops in Iran accelerated social change and it roused nationalist sentiment in the country. In 1946, Hossein Gol-e-Golab published the nationalist song Ey Iran; it was reportedly inspired by an incident during the war in which Golab witnessed an American GI beating up a native Iranian greengrocer in a marketplace dispute.
Strategic need for supply to the USSR
After the British were pushed off the continent, Germany was essentially without any opposition in Europe. British and American leaders sought to eventually establish another front opened up to engage the German military. In June 1941 Hitler launched the invasion of the USSR.
The western Allies made the strategic decision to provide Stalin with significant material support to ensure that the USSR could continue to engage a significant portion of the German military.
Agreements were created (protocols) which clearly defined the type and amount of materiel that would be delivered within a given time frame. Due to German military action on the Northern route and the fact that it could not be traversed during part of the year, the US was unable to meet the demands of the first protocol. This caused increasing pressure on the Allies to develop the route using the Persian Corridor.
The Allies delivered all manner of materiel to the Soviet Union ranging from Studebaker US6 trucks to American canned food. Most of the supplies transiting through the Persian Corridor arrived by ship at various ports in the Persian Gulf and were then carried northwards by railroad or in long truck convoys. Some goods were later reloaded onboard ships to cross the Caspian Sea and others continued their journey by truck.
The United States Army forces in the corridor were originally under the Iran-Iraq Service Command - later renamed the Persian Gulf Service Command (PGSC). This was the successor to the original United States Military Iranian Mission, which had been put in place to deliver Lend-Lease supplies before the United States had entered the World War. The mission was originally commanded by Colonel Don G. Shingler, who was then replaced late in 1942 by Brigadier General Donald H. Connolly. Both the Iran-Iraq Service Command and the PGSC were subordinate to the U.S. Army Forces in the Middle East (USAFIME). PGSC was eventually renamed simply the Persian Gulf Command.
The Allied supply efforts were enormous. The Americans alone delivered over 16.3 million tonnes to the Soviets during the war, via three routes, including the Arctic Convoys to the ports of Murmansk and Archangelsk. Also, Soviet shipping carried supplies from the west coast of the United States and Canada to Vladivostok in the Far East, since the Soviet Union was not at war with Japan until August 1945. The Persian Corridor was the route for 4,159,117 long tons (4,225,858 metric tonnes) of this cargo. However, this was not the only allied contribution via the Persian Corridor. About 7,900,000 long tons (8,000,000 metric tonnes) of shipborne cargo from Allied sources were unloaded in the Corridor, most of it bound for the USSR - but some of it for British forces under the Middle East Command, or for the Iranian economy, which was sustaining the influx of tens of thousands of foreign troops and Polish refugees. Also, supplies were needed for the development of new transportation and logistics facilities in Persia and in the Soviet Union. The tonnage figure does not include transfers of warplanes via Persia.
Supplies came from as far away as Canada and the United States, and those were unloaded in Persian Gulf ports in Iran and Iraq. Once the Axis powers were cleared from the Mediterranean Sea in 1943 - with the Allied capture of Tunisia, Sicily, and southern Italy - cargo convoys were able to pass through the Mediterranean, the Suez Canal, and the Red Sea to Iran for shipment to the USSR.
The main ports in the Corridor for supplies inbound to Iran were: in Iran,
The main overland routes were from the ports to Tehran, and then
Important smaller ports and transit points on the routes included:
- Bandar Anzali
- Bandar Abbas
- Bandar-e Shah (now Bandar Torkoman)
- Amir Abad port
- BIK port
- Fereydunkenar 
Cargo was principally handled by special British and American transportation units from the nations' respective combat service support branches, such as the Royal Army Service Corps and the United States Army Quartermaster Corps. Many Allied civilian workers, such as stevedores and railway engineers, were also employed on the corridor. Many skilled engineers, accountants and other professionals who volunteered or were drafted into the armed services were made warrant officers to help oversee the complex supply operations.
In addition to providing logistical support to the Iranians, the Allies offered other services as well. The Americans in particular were viewed as more neutral since they had no colonial past in the country as did the British and Soviets. The Americans contributed special expertise to the young Shah's government. Colonel Norman Schwarzkopf, Sr., who at the outbreak of the war was serving as superintendent of the New Jersey State Police was in August 1942 put in charge of training the Imperial Iranian Gendarmerie (his son, Norman Schwarzkopf, Jr., would command coalition forces fifty years later during the Persian Gulf War.)
To help operate trains on the demanding Trans-Iranian Railway route, the US supplied large numbers of ALCO diesel locomotives, which were more suitable than steam locomotives. About 3000 pieces of rolling stock of various types were also supplied.