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A couple of decades before Britain sat Mohammad Reza Shah Pahlavi on the Iranian throne and several decades before they helped engineer the coup that kept him in power, British operatives were also responsible for the Iranian coup that enthroned his father, Reza Shah Pahlavi (d. 1944), and instituted 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, which the Pahlavis succeeded, covered 7 rulers and 136 years. But who's counting?
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 replaced (sort of; the name was different) imperial Russia. Iran (or Persia, if you prefer) was never a colony, as such, but it still lost most of its autonomy to the two empires, who viewed Iran as part of their mutual frontier and informally divided 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. If anything, that accord left Iran, which had in 1905 transitioned (well, sort of) from an absolute monarchy to a constitutional one, in worse shape than before. Instead of being informally divided into two spheres of influence, Iran was now…formally divided into the same two spheres of influence. The more things change, etc. As you might imagine, a monarch trying to rule a kingdom that two much more powerful empires had divvied up between them was inevitably going 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 out the window. Shortly after the revolution and Russia’s withdrawal from the war, a British-led force moved north from Iran to capture 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 this British intervention, and so in 1920 they organized a force of Iranian and Caucasian Communists to invade Iran, whence it had originated, from the north (with Red Army support, of course).
Fearing the 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 senior British military officer in Iran, tasked Reza Khan, commander of the country's Cossack Brigade, 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 Khan was the first Iranian to command it. And the last, as it turns out, as the Cossack Brigade was subsequently dissolved and became the core of the new Iranian army.)
Reza and his men had no trouble marching (ok, to be fair, I don’t actually know if they marched) into Tehran in February 1921 and forcing Ahmad Shah to appoint a new government, though there were a number of local uprisings that popped up in the wake of that action that he had to put down. This government had Reza serving merely as minister of war. But in that capacity, Reza named himself commander-in-chief of the army, so there was no question who was the real power in the new regime. The new government signed a treaty of friendship with Moscow, in which the Russians agreed to withdraw from Iran but reserved the right—acknowledged by the Iranians—to invade again on national security grounds.
Reza spent the next couple of years putting down several local revolts that had been sparked by the coup. When he’d finally done that, he returned to Tehran in 1923 and accepted the post of prime minister in triumph. Ahmad Shah decided to remove himself from the situation before Reza could 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 Iranian 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 Reza appears to have supported this idea. But the idea was opposed by both the Iranian 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. They suggested that Reza keep the monarchy in place but under constitutional limits. How’d that work out for them, you ask? Instead of a possibly secular republic led by a possibly dictatorial figure, they 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 develop the idea of Iranian nationhood in ways that the Qajars—who in reality 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, not “Persia,” was the name that the Iranian people had always used for their homeland.
Reza Shah also took pains to divest Iran of its heavily dependent relationship with the British Empire, and this wound up costing him his job in the end. He knew that the Brits had put him into power and was acutely aware that they could remove him from power just as easily. So he brought in many non-British advisers, for example, and in particular gave concessions to German firms at the expense of British ones. Perhaps you can see where this is going. Reza was no Nazi (in fact he treated Iran’s Jewish population pretty well), but when World War II broke out his German sympathies (which were more anti-British sympathies than anything else) made him suspect, and the Allies began to worry about Iranian oil feeding the Axis war effort.
Reza was quick to declare Iran’s neutrality. But when the Germans invaded the Soviet Union in 1941, Britain and the Soviets saw Iran as a vital corridor for getting supplies from the former to the latter, and Reza’s neutrality became a big problem for them. A surprise 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.