Decades before they helped engineer the coup that restored Mohammad Reza Shah Pahlavi to power, the British were responsible for engineering another Iranian coup that (eventually) enthroned his father, Reza Shah Pahlavi (d. 1944), and thus founded the Pahlavi “Dynasty.” I put “dynasty” in quotes because it was just the two of them over the 54 years it was in power–by contrast, the Qajar Dynasty that the Pahlavis succeeded covered 7 rulers (two of whom ruled for more than 35 years each) and 136 years.
Anyhoo, the 1921 Persian coup was of a piece with the political and military maneuvers that had marked the 19th century “Great Game” competition for Asian supremacy between the Russian and British Empires, only in this case Soviet Russia had (it wasn’t the USSR yet) replaced the imperial Russians. Iran (or Persia, if you prefer) was never a colony, as such, but it gave up considerable autonomy to the two empires, who viewed Iran as part of the dividing line between their Asian possessions and informally split the country into spheres of influence–Russian in the north, British in the south. The Great Game ended with the Anglo-Russian Convention of 1907, which settled things between the two empires but left Iran, which had in 1905 transitioned from an absolute monarchy to a constitutional one, as a single nation that was now formally divided into those two spheres of influence. As you might imagine, any monarch trying to rule a kingdom that two much more powerful empires had divided among themselves was likely to be pretty ineffectual, and Ahmad Shah Qajar (d. 1930) probably wouldn’t have been that strong a ruler even in the best of circumstances.
Then World War I happened, and the Russian Revolution happened, and that whole Anglo-Russian partnership went right out the window. Shortly after the revolution and Russia’s withdrawal from the war, a British-led force moved north from Iran to take and hold Baku, in Azerbaijan, so as to prevent its nearby oil fields from falling into Ottoman hands. Britain sent another force across the Caspian to support a Menshevik resistance movement in modern Turkmenistan. Surprisingly, the Bolsheviks didn’t take too kindly to these British moves, and so in 1920 they organized a force of Iranian and Caucasian Communists to invade the northern part of Iran (with Red Army support, of course) and continue on toward Tehran.
Fearing the total loss of Iran to the Russians, British officials inside the country decided that a military coup was necessary to protect their interests. General Edmund Ironside, the top British officer in Iran, tasked Reza Khan, commander of the country’s Cossack Brigade and the country’s most prominent military leader, with entering Tehran and overthrowing the current government. It’s likely that Ironside provided supplies and payment to Reza and his men in exchange for their participation in the coup. The Cossack Brigade, ironically enough, had been a gift from Russia to the Qajars in 1879, modeled after the Cossack cavalry units in the Russian Army and commanded by loaned Russian officers. Reza was the first Iranian to command it. And the last, as it turns out, as he would subsequently dissolve the brigade and use its personnel to seed the core of a new Iranian army.
Reza and his men had no trouble entering Tehran in February 1921 and forcing Ahmad Shah to appoint a new government. Reza initially served “merely” as minister of war, but he’d also arranged his own appointment as commander-in-chief of the army so there was no question who the real power in Iran was. The new government signed a treaty of friendship with the Soviets, in which they agreed to withdraw from Iran in exchange for an acknowledgement by Tehran that Russia (and later the USSR) had the right to intervene in Iran again if they felt their security depended on it.
Reza spent the couple of years putting down several local revolts that had kicked off in the aftermath of the coup. When he’d finally done that, in 1923, he returned to Tehran and accepted the post of prime minister in triumph. Ahmad Shah decided to remove himself from the situation before Reza decided to have him removed, so he took an extended vacation to Europe that lasted the rest of his life. Reza effectively ruled as a dictator from this point on, but it wasn’t until December 12, 1925 that the Majles (parliament) voted to depose Ahmad and crown Reza the new Shah (which happened in a ceremony on December 15).
At the time, there was a movement brewing to transform Iran into a republic along the Turkey/Atatürk line, and there’s some evidence that Reza was sympathetic to this idea. But he was opposed in this by both the religious establishment, which feared that Iran would become as secular as Atatürk’s Turkey looked to be, and by some reformers, who feared that Reza would become a quasi-dictator, like, well, Atatürk looked to be. They suggested that Reza keep the monarchy in place but under constitutional limits. We know how this all worked out. Instead of a possibly secular republic led by a possibly dictatorial figure, Iran got a definitely secular absolute monarch. Oh well.
Although he became increasingly more despotic and repressive as time went on, Reza Shah was an important figure in the development of modern Iran. To be fair, if he would have done nothing else apart from clearing out the rotting husk of the Qajar Dynasty, that still would have been a significant accomplishment. But Reza helped to craft the idea of Iranian nationhood in ways that the Qajars, who in the end were Turks–albeit very Persianized Turks–couldn’t and wouldn’t have done. Reza invested in railroads, education, and manufacturing, built a powerful modern army, reformed Iran’s weak judicial system, weakened divisive tribal authorities, and promoted the education and professional development of women. All very Atatürk-ian. He’s also the reason why the rest of the world calls Iran “Iran” today, because he explained to the League of Nations in 1935 that Iran was the name that the Iranians had always used for their homeland, not “Persia.”
Reza Shah also took pains to move Iran away from its heavily dependent relationship with the British Empire. He knew that the Brits had put him in power and was acutely aware that they could force him out of power just as easily, and so he took steps to reduce British influence. He brought in many American advisers, for example, and gave concessions to German firms at the expense of British ones. The latter policy helped seal his fate. Reza was no Nazi (in fact he treated Iran’s Jewish population pretty well), but the ties he’d cultivated with German firms made him suspect in British eyes when World War II broke out, and the Allies began to worry about strategic considerations like Iranian oil feeding the Axis war effort.
Ironically, if it was British-Soviet post-World War I hostility that put Reza Shah in power, it was the British-Soviet World War II alliance that toppled him out of it. Reza was quick to declare Iran’s neutrality in the war. But when the Germans invaded the Soviet Union in 1941, the Brits saw the Iranian route as a vital corridor for getting supplies to their new Soviet pal. Reza’s neutrality thus became a big problem for them, and for the Soviets (who desperately needed those supplies). A surprise joint Soviet-British invasion of Iran in late-August 1941 caught the overwhelmed Iranians completely by surprise, and Reza Shah was forced to abdicate in favor of his much more compliant son.