Relations date back more than a millennium between these two former and would-be future empires, whose geographic vicinity has varied over the centuries, depending on the reach of the respective domains. Official diplomatic relations were established in 1521 between the Grand Duchy of Moscow and the Persian Empire, with commercial and diplomatic ties ebbing and flowing until the late 19th century, when Russia and Britain sought to control parts of Iran. The Russian Revolution of 1917 would eventually lead to the expansion of the Soviet Union to include Azerbaijan, which meant that the USSR and Iran would share a border for decades. During the Cold War, relations between the USSR and Iran were ambiguous: Politically, the Shah's Iran was oriented toward the United States. But economically, Moscow and Tehran maintained solid trade relations. Soviet leader Mikhail Brezhnev greeted the 1979 Islamic Revolution in Iran favorably, as it targeted Tehran's alliance with Washington; yet in the Iran-Iraq war, the USSR supported Iraq even while continuing to trade and build infrastructure and energy facilities in Iran. The withdrawal of Russian troops from Afghanistan markedly improved Iranian-Russian contacts, and Moscow has also been Iran's key nuclear energy partner since 1992. More recently, Russia also helped Iran solve some of its regional military problems: In 2015, Putin sent air power to Syria to prevent the collapse of President Bashar al-Assad, whom Iran actively supported. The operations required Russian and Iranian militaries to work closely together, including Moscow’s air force covering Iranian fighters and their proxy forces in ground operations. While the regimes of Putin and Supreme leader Ali Khamenei share a prime enemy in the United States, Tehran had held off in supplying any material support after the Russian invasion of Ukraine. But that has changed with the arrival of both the deadly drones and Iranian military personnel to train Russian troops.