The British force broke through the border at the town of Qasr-e Shirin and moved into the Naft Shahr oilfield with little opposition. The British stated that the operation had been carried out with minimum losses for the Iranians but British troops faced a determined defence by 2,000 Iranians as they tried to capture the town of Gilan-e Gharb 30 kilometres inside of Iran, which if successful would block the British from moving through the steep Pai Tak mountain pass. The RAF provided close air support and was involved in several dogfights with Iranian aircraft. Six Iranian fighters were shot down and several others damaged, for no loss, ensuring air superiority. The RAF also bombed several local towns and dropped leaflets urging surrender.
The British captured Gilan-e-Gharb and attacked Iranian forces who defended the town of Sarpol-e-Zahab. With overwhelming firepower and decreasing Iranian morale, the British captured that town, scattering the remaining defenders. The Pai Tak Pass, and the road to Kermanshah and eventually Tehran was open. The armoured columns began to secure the pass and the areas around it. The British forces moved along the Kermanshah highway towards the city of Shahabad. There was little Iranian resistance but some trees were cut down and a section of the road was even dynamited, which delayed the British forces for several hours.
The main Iranian forces in the region consisted of the 5th and 12th infantry divisions of 30,000 troops with supporting artillery at Kermanshah and Sanandaj. They were all light infantry (as the mechanised and armour had been stretched thin fighting on multiple fronts). The British reached the outskirts of Shahabad in the early morning hours of 28 August after delays. At the village of Zibri, they faced a strong Iranian garrison willing to put up a fight which caused the British several casualties but with poor Iranian leadership and overwhelming British firepower, resistance crumbled and the British took Shahabad on the morning of the same day. By 29 August, the British had reached the town of Kerend and were within 3 kilometres of Kermanshah and the Iranian commanders were told of the ceasefire order and stood down. The defenders declared Kermanshah an open city and the British entered on 1 September. They also entered Sanandaj peacefully and eventually Qazvin, which had already been captured by the Red Army.
The Soviet forces attacked on 25 August and Iranian airbases were destroyed by preliminary air attacks. The Soviets attacked using three armoured spearheads, totalling over 1,000 tanks and motorised infantry; the Iranians had no tanks in the area. The first force, consisting of the 47th Army broke through the border and moved from the Azerbaijan SSR into Iranian Azerbaijan. They moved towards Tabriz and Lake Urmia. They captured the Iranian city of Jolfa. An Iranian reconnaissance aircraft discovered the forces south of Jolfa moving towards Marand. It was possible for the Iranian 3rd Division under General Matboodi to move motorised infantry towards Shibli to halt the breakthrough, but due to being taken by surprise, he failed to make the proper counter-attack. He also failed to destroy the bridges and highways with explosives, allowing the Soviets to rapidly move through the region. Five Iranian bombers were intercepted trying to attack the Soviet positions around Jolfa.
The 53rd Army crossed the border and moved towards the city of Ardabil, defended by Iran's 15th Division led by Brigadier-General Qaderi. Two Iranian regiments began to move towards the town of Nir to confront the invaders. Despite having a solid force and well-motivated troops, Qaderi jumped into his car and abandoned his troops. He sabotaged the defence even further by ordering the supply trucks delivering food, weapons and artillery to unload their weapons to make way for his personal belongings. The Soviets bypassed Nir and moved south. Ardabil was bombed by the Soviet Air Force and received minor damage to its barracks. Cut off and bypassed, both the Iranian 15th Division in Ardabil and the 3rd Division in Tabriz began to collapse. Despite that, the regular troops tried to maintain order and began to march towards the enemy without many of their commanders. However, lacking food, supplies and ammunition, the troops were forced to abandon much of their heavy equipment. Heavy pockets of resistance remained, with some desperate fighting until the end. They were unsurprisingly beaten by the Soviets, who 26 August had occupied Iranian Azerbaijan (including Tabriz and Ardabil).
On 25 August, the Soviet attack against Gilan Province began with their Caspian Sea Flotilla, led by Rear-Admiral Sedelnikov. The flotilla consisted of more than a dozen patrol boats, destroyers, multiple anti-aircraft barges and landing craft. Facing them were three Iranian gunboats. Meanwhile, the 44th Army crossed the border and moved into Gilan Province. They moved along the Astara highway and the main coastal highway (Jadeh-e-Shomal). Heavy Iranian forces in the area made the naval landing force secure Iranian cities, which were then joined by the land forces. The flotilla landed troops and rapidly captured the border city of Astara. The landing force boarded their ships and moved towards their next targets.
The main objective of the attack was to capture Iran's Caspian Sea port of Bandar Pahlavi. The Iranian forces in Gilan, led by General Iranpour, made their stand at the provincial capital of Rasht and Bandar Pahlavi and offered a stubborn resistance.
The Iranian forces sank barges at the entrance to Pahlavi harbour, and lacking coastal artillery, moved a battery of 75mm guns to the area. The Iranians fought desperately, and despite Soviet superiority, the Iranians prevented them from landing. The Iranians were careful to not fire their guns while Soviet aircraft flew overhead to prevent them from disclosing their location. Soviet aircraft were kept at bay by 47mm anti-aircraft artillery on Iranian barges.
The next day, however, the Soviet Air Force moved into action, using many heavy bombers. In groups of 4 aircraft each, their bombers attacked military positions and civilian targets throughout Gilan, including Bandar Pahlavi and Rasht. At least 200 civilians were killed during the bombings. The bombings also destroyed many Iranian positions, and resistance was finally crushed by the 44th Army advancing from land, capturing both cities. Fighting was very intense, and the Soviets took their heaviest casualties of the invasion here. However, lacking armour and air power, the Iranians could not stop the enemy. On 28 August, they were forced to surrender. Nevertheless, some Iranian forces refused to accept defeat, and retreated to Ramsar to continue fighting. Their efforts were undercut when the Iranian government announced a ceasefire the next day. By then, the Soviet forces had reached the city of Chalus, meaning that they could cross the Chalus Highway (Jadeh-e-Chalus) and reach Tehran across the Alborz Mountains.
Meanwhile, the Soviet invasion force in Iranian Azerbaijan had moved south. The 47th Army had been delayed in the Jolfa area when three individual Iranian soldiers managed to block an important bridge until they ran out of ammunition and were killed. The Soviets did not use artillery for fear that they would damage the bridge and delay their advance further. The 47th Army moved south, capturing Dilman (100 kilometres west of Tabriz) and then Urmia (Oromiyeh), ostensibly to block the escape of "German agents". The latter was defended by only a few snipers. The Soviets responded by bombing targets in the city, killing over a dozen people and wounding many others, and much of the city's bazaar was burned.
Meanwhile, the 53rd Army moved south of Ardebil towards the Tehran-Karaj-Tabriz highway, capturing the city of Meyaneh (Mianeh) and moving southeast towards Qazvin and Tehran by 27–28 August. Iran's 15th and 3rd divisions had already been bypassed and defeated, and there was only sporadic resistance against the Soviets. The Soviet armoured spearhead drove down the highway and poised to take Qazvin on the 29th (151 kilometres from Tehran), followed by Saveh and Qom, south of Tehran, cutting the main Tehran-Saveh-Persian Gulf highway and cutting Iran effectively in two. But the Iranians accepted the ceasefire on 29 August, and the Soviets entered the now "open city" on 30 August. At the same time, elements of the 53rd Army captured the city of Hamadan. One civilian (a small child) was killed in a small bombing raid, and the sporadic resistance was defeated. They stopped their advance on 1 September and did not move further towards Tehran from Qazvin in light of negotiations with Iran's government.
On 25 August, the Soviet Army invaded northeastern Iran from the Turkmenistan SSR. Details of this invasion were not nearly as extensive as details of the others. The Soviet invasion force had to cross mountainous terrain, and its goals were to recruit new troops from the Turkmen Sahra, assemble with the Soviet troops and to capture the city of Mashhad, the second largest city in Iran.
Defending Mashhad and Khorasan province was Iran's 9th Infantry Division, totalling 8,000 troops. They were light infantry, and it was unlikely that they could defend against the more numerous Soviet forces with armour and air power. The Soviet Air Force bombed Mashhad Airport, destroying many Iranian fighter aircraft, along with numerous military barracks. The Soviet forces advanced in three columns across the border. There was heavy fighting for three days, and by 28 August, the Iranians had been driven back after taking heavy casualties. Mashhad fell to the Soviets the same day
By 28–29 August, the Iranian military situation was in complete chaos. The Allies had complete control over the skies of Iran, and large sections of the country were in their hands. Major Iranian cities (such as Tehran) were suffering repeated air raids. In Tehran itself, the casualties had been light, but the Soviet Air Force dropped leaflets over city, warning the population of an upcoming massive bombing raid and urging them to surrender before they suffered imminent destruction. Tehran's water and food supply had faced shortages, and soldiers fled in fear of the Soviets killing them upon capture. Faced with total collapse, the royal family (except the Shah and the Crown Prince) fled to Isfahan.
The collapse of the army that Reza Shah had spent so much time and effort creating was humiliating. Many of the military generals had behaved incompetently or secretly sympathised with the British and ended up sabotaging the Iranian resistance. The army generals met in secret to discuss surrender options. When the Shah learned of the generals' actions, he beat the head of the armed forces General Ahmad Nakhjavan with a cane and physically stripped him of his rank. He was nearly shot by the Shah on the spot, but at the insistence of the Crown Prince, he was sent to prison instead.
The Shah ordered the resignation of the pro-British Prime Minister Ali Mansur, whom he blamed for demoralising the military. He was replaced with Mohammad Ali Foroughi, a former prime minister. The Shah ordered the Iranian military to end resistance and order a ceasefire. He entered into negotiations with the British and Soviets.
Foroughi was an enemy of Reza Shah (he was forced into retirement in earlier years for political reasons, and his son was executed by firing squad). When he entered into negotiations with the British, instead of negotiating a favourable settlement, Foroughi implied that both he and the Iranian people wanted to be "liberated" from the Shah's rule.
The British and Foroughi agreed that for the Allies to withdraw from Iran, the Iranians would have to assure that the German minister and his staff should leave Tehran; the German, Italian, Hungarian and Romanian legations should close and all remaining German nationals (including all families) to be handed over to the British and Soviet authorities. The last order would mean almost certain imprisonment or, in the case of those handed to the Soviets, possible death. Reza Shah delayed on the last demand. Instead, he planned the secret evacuation of all German nationals from Iran. By 18 September, most of the German nationals had escaped via the Turkish border.
In response to the Shah's defiance, the Red Army on 16 September moved to occupy Tehran. Fearing execution by the communists, many people (especially the wealthy) fled the city. Reza Shah, in a letter handwritten by Foroughi, announced his abdication, as the Soviets entered the city on 17 September. The British wanted to restore the Qajar Dynasty to power because they had served British interests well prior to Reza Shah's reign. However, the heir to the throne, Hamid Hassan Mirza, was a British citizen who spoke no Persian. Instead (with the help of Foroughi), Crown Prince Mohammad Reza Pahlavi took the oath to become the Shah of Iran.
Reza Shah was arrested before he was able to leave Tehran, and he was placed into British custody. He was sent to exile as a British prisoner in South Africa, where he died in 1944. The Allies withdrew from Tehran on 17 October and Iran was partitioned between Britain and the Soviet Union for the duration of the war, with the Soviets stationed in northern Iran and the British south of Hamadan and Qazvin.
The Persian Corridor became the route for a massive flow of supplies (over 5 million tons of matériel) to the Soviet Union and also the British in the Middle East. At the end of August 1942, German intelligence agents spread leaflets in Tabriz and other cities; an underground fascist organisation called Melnune Iran, was founded. Agents of Melnune Iran instigated anti-government protests in the lake Urmia region. The Bakhtiari and Qashqai peoples carried out armed resistance against the new government.
Shah Reza Pahlavi
At the Tehran Conference in November of that year, Roosevelt, Winston Churchill and Joseph Stalin reaffirmed their commitment to Iranian independence and territorial integrity, with a willingness to extend economic assistance to Iran. The treaty ruled that Iran was not considered to be "occupied" by the Allies but instead a member of the Allied Powers.
The effects of the war were very disruptive for Iran. Much of the state bureaucracy had been damaged by the invasion and food and other essential items were scarce. The Soviets appropriated most of the harvest in northern Iran, leading to food shortages for the general public. The British and Soviet occupiers used the delivery of grain as a bargaining chip and the food crisis was exacerbated because foreign troops needed to eat and use the transport network to move military equipment.
The British pressured the Shah to appoint Ahmad Qavam to be the prime minister, who proceeded to mismanage the entire food supply and economy. In 1942, bread riots took place in Tehran, Martial law was declared and several rioters were killed by the police. inflation increased by 450 percent, imposing great hardship on the lower and middle classes. In some areas there were famine deaths but there was virtually no armed resistance against the occupation.
In 1943, 30,000 Americans helped to man the Persian Corridor and 26–34 percent of the supplies sent to the Soviet Union under the Lend-Lease Act were sent through Iran. The Americans also assuaged Iranian fears of colonisation by the two powers by confirming that they would respect the independence of Iran. The US also extended Lend-Lease assistance to Iran and began to train the Iranian army.
Arthur Millspaugh became the finance minister of Iran but ran into much opposition trying to direct Iranian finances.
There were two notable German attempts to undertake operations against the Allies in 1943. In the summer of 1943, Abwehr's Operation Francois was an attempt to use the dissident Qashqai people in Iran to sabotage British and American supplies bound for the Soviet Union. Also in 1943, Operation Long Jump was an unsuccessful German plot to assassinate the "Big Three" Allied leaders (Stalin, Churchill and Roosevelt) at the Tehran Conference.
During the three years of occupation, Joseph Stalin had expanded Soviet political influence in Azerbaijan and Iranian Kurdistan in northwestern Iran, as well as in Iran founding the communist Tudeh Party of Iran. The Soviets had attempted during their occupation to stir tensions between the tenant-farmers and the landlords (known in Iran as arbabs). On 12 December 1945, after weeks of violent clashes a Soviet-backed separatist People's Republic of Azerbaijan was founded. The Kurdish People's Republic was also established in late 1945. Iranian government troops sent to reestablish control were blocked by Red Army units.
When the deadline for withdrawal arrived on 2 March 1946, six months after the end of World War II hostilities, the British began to withdraw, but Moscow refused, citing "threats to Soviet security". Soviet troops did not withdraw from Iran proper until May 1946, following Iran's official complaint to the newly formed United Nations Security Council, which became the first complaint filed by a country in the UN's history, and a test for the UN's effectiveness in resolving global issues in the aftermath of World War II.
However, the UN Security Council took no direct steps to pressure the Soviets to withdraw