The Anglo-Soviet invasion of Iran, also known as Anglo-Soviet invasion of Persia, was the invasion of the Empire of Iran during the Second World War by Soviet, British and other Commonwealth armed forces. The invasion lasted from 25 August to 17 September 1941 and was codenamed Operation Countenance. Its purpose was to secure Iranian oil fields and ensure Allied supply lines (see Persian Corridor) for the USSR, fighting against Axis forces on the Eastern Front. Though Iran was neutral, the Allies considered Reza Shah to be friendly to the Axis powers, deposed him during the subsequent occupation and replaced him with his young son Mohammad Reza Pahlavi.
In 1925, after years of civil war, turmoil and foreign intervention, Persia was unified under the rule of Reza Khan, who crowned himself to become Reza Shah that same year. Later, in 1935, he asked foreign delegates to use the term Iran, the historical name of the country, used by its native people, in formal correspondence. He set on an ambitious program of economic, cultural, and military modernisation. Iran, which had been a divided and isolated country under the rule of the Qajar Dynasty, was now rapidly evolving into a modern industrial state. Reza Shah also made many improvements, such as building infrastructure, expanding cities and transportation networks, and establishing schools. He also set forth on a policy of neutrality, but to help finance and support his ambitious modernisation projects, he needed the help of the west.
For many decades, Iran and the German Empire had cultivated ties, partly as a counter to the imperial ambitions of Britain and the Russian Empire (and later, the Soviet Union). Trading with Germany appealed to Iran because the Germans did not have a history of imperialism in the region, unlike the British and Russians. The Iranian government didn't support the anti-semitism of Nazis. Iranian embassies in occupied European capitals by Germans rescued over 1,500 Jews and secretly granted them Iranian citizenship, allowing them to move to Iran.
The British began to accuse Iran of supporting Nazism and being pro-German. Although Reza Shah declared neutrality at an early stage of the Second World War, Iran assumed greater strategic importance to the British government, which feared that the Abadan Refinery (of the UK-owned Anglo-Iranian Oil Company) might fall into German hands; producing eight million tons of oil in 1940, the refinery was a crucial part of the Allied war effort. Tensions with Iran had been strained since 1931 when the Shah cancelled the D'Arcy Concession, which gave the Anglo-Iranian Oil Company the exclusive right to sell Iranian oil, with Iran receiving only 10 percent (possibly 16 percent) of the revenue or of the profits.
Following Operation Barbarossa, the German invasion of the Soviet Union in June 1941, Britain and the Soviet Union became formal Allies, providing further impetus for an Allied invasion. With the German Army steadily advancing through the Soviet Union, the Persian Corridor formed by the Trans-Iranian Railway was one of the easiest ways for the Allies to get Lend-Lease supplies to the Soviets, sent by sea from the United States. British and Soviet planners recognised the importance of that railway and sought to control it. As increasing U-boat attacks and winter ice made convoys to Arkhangelsk dangerous, the railway became an increasingly attractive route. The Soviets wanted to make Iranian Azerbaijan and the Turkmen Sahra part of the Soviet Union or even turn Iran into a communist state. The two Allied nations applied pressure on Iran and the Shah, which led only to increased tensions and anti-British rallies in Tehran. The British described the protests as being "pro-German". Iran's strategic position threatened Soviet Caucasian oil and their armies' rear and a German advance would threaten British communications between India and the Mediterranean.:215–216