During World War II, the Soviets and the British signed a pact to launch a massive land, air and naval assault on Iran, a neutral country during World War II.
This was considered to be a preemptive invasion, due to the good relationship between the Empire of Iran and NS-Germany at that time. Iran is a country full of rich resources, especially oil, and its leader - King Reza Shah - was friendly to the Axis powers.
This friendship between Iran and Germany was a product of decades. Iran preferred the German Empire over the British Empire and the Russian Empire because the Germans did not have imperial history in the region.
After Operation Barbarossa, in which Germany invaded the Soviet Union and made tremendous successes initially, Iran expected Germany to win the war. As a result of this invasion, Britain and the USSR became formal allies.
They feared that if Germany would reach the borders of Iran, the Abadan Refinery would fall in German hands, which would result in a massive back-up for German war-effort and resources.
The rich oil fields were of vital importance to the Allied war effort, and were conquered so Germany could not use it.
Thus, in 1941 the British and the Soviets invaded the Empire of Iran. The Battle lasted from the 25th of August till 17th of September, 1941. Over 900 soldiers and 200 civilians had died in the battles, with most casualties on the Iranian side. Once Britain and the USSR had conquered Iran, they forced the abdication of King Reza Shah, and instead put into power his young son, Mohammad Reza Pahlavi.
Following their victory in Iran, the British and the Soviets demanded the expelling of German residents in Iran. Approximately 3000 Germans lived in Iran during that time and had big influence on the Iranian strategic government industries and the transport and communications network.
The British also used Iran as a passageway to ship massive quantities (over 5 million tons) of goods towards the USSR through the lend-lease program. Iran was be used as an allied wedge to halt the German invasion of the USSR. This greatly reinforced the Russian war-effort against Germany and hastened Germany's defeat.
British Lend-Lease supplies to the USSR included planes, tanks, munitions, food and more.
Following Germany's defeat, Iran was still under occupation by Soviet and other Allied forces. Britain began to withdraw their troops from Iran six months after the end of World War II, but Stalin refused to do the same. The Iranian government filed the first complaint to the newly formed United Nations, but the organization took no real effort to pressure the Soviets to leave. It was not until 1946 that the Soviets began diminishing their presence.