On 25 August 1941, Britain and the Soviet Union invaded Iran. The occupation secured the Allies continued access both to Iranian oilfields and to the so-called Persian Corridor.
The Allies' military supply line stretched from the Persian Gulf through Iran and into the Soviet Union, which was under heavy attack by Nazi Germany.
Although a Treaty of Alliance between the British and Soviet governments promised "to respect the territorial integrity", the invasion and subsequent occupation of neutral Iran had undeniable, catastrophic impacts on the country.