The Anglo-Soviet invasion of Iran, also known as Anglo-Soviet invasion of Persia, was the invasion of the Imperial State 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 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 did not support the antisemitism 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 World War II, 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.