On Christmas Eve 1979, the Soviet Union began an invasion of Afghanistan, its Central Asian neighbor to the south. First, it air-dropped elite troops into principal Afghan cities. Soon after, it deployed motorized divisions across the border. Within days, the KGB, which had infiltrated the Afghan presidential palace, poisoned the president and his ministers, helping launch a Moscow-backed coup to install a new puppet leader, Babrak Karmal. The invasion triggered a brutal, nine-year-long Afghan civil war.

By the time the last Soviet troops pulled out in early 1989, rumbling back across the ironically named “Friendship Bridge,” the conflict had cost the lives of an estimated 1 million civilians and some 125,000 Afghan, Soviet and other combatants. The war wreaked havoc not only on Afghanistan, but on the Soviet Union, whose economy and national prestige took a severe drubbing. The military misadventure would contribute significantly to the USSR’s later collapse and breakup.

So why did Moscow do it?

From the early 19th century onward, Afghanistan became a geopolitical pawn in what came to be known as “The Great Game” between the empires of Tsarist Russia and Great Britain. Fearful that Tsarist Russia’s expansion into Central Asia would bring it perilously close to the border of India, their imperial jewel, Britain fought three wars in Afghanistan to maintain a buffer against Russian encroachment.

Neither the Russian Revolution of 1917 nor the end of British colonial rule in India altered Afghanistan’s geopolitical significance. In 1919, the year Afghans won independence to conduct their own foreign policy, the Soviet Union became the first country to establish diplomatic relations with Afghanistan—which, in turn, was one of the first to formally recognize the Bolshevik government.

Over subsequent decades, the USSR offered both economic and military aid to a neutral Afghanistan. When the British empire declined after World War II and the United States emerged as a dominant world power, Afghanistan remained on the Cold War front lines.

In 1973, Afghanistan’s last king was ousted in a coup by his cousin and brother-in-law, Mohammed Daoud Khan, who proceeded to establish a republic. The USSR welcomed this shift to the left, but their delight soon faded as the authoritarian Daoud Khan refused to be a Soviet puppet. During a private 1977 meeting, he told Soviet leader Leonid Brezhnev he would continue to employ foreign experts from countries beyond the USSR. “Afghanistan shall remain poor, if necessary, but free in its acts and decisions.” Unsurprisingly, Soviet leaders disapproved. In 1978, the communist People’s Democratic Party of Afghanistan (PDPA) overthrew Daoud Khan in what became known as the Saur Revolution. Daoud Khan and 18 family members died.

Despite Afghanistan’s nominally communist leadership, Soviet leaders still couldn’t relax. The new PDPA regime, divided and unstable, faced fierce cultural resistance from conservative and religious leaders, and opposition throughout much of the Afghan countryside to the communists’ radical agrarian reforms. In the fall of 1979, revolutionary Hafizullah Amin orchestrated an internal PDPA coup that killed the party’s first leader and ushered in his brief, but brutal reign. National unrest soared, and Moscow’s hand-wringing intensified.

Afghanistan’s chaos alarmed Soviet leadership primarily because it increased the odds that Afghan leaders might turn to the United States for help. Top Politburo members warned Brezhnev in late October 1979 that Amin sought to pursue a more “balanced policy” and that the United States was detecting “the possibility of a change in the political line of Afghanistan.”

Only weeks later, KGB head Yuri Andropov joined the USSR’s foreign minister Andrei Gromyko and its defense minister Dmitri Ustinov in sounding the alarm. They persuaded Brezhnev that even if the Americans weren’t actively trying to undermine Soviet influence in Afghanistan, Amin’s ruthless but unstable regime would create weaknesses the U.S. could later exploit. Moscow, they argued, would have to act.

Those warnings likely fell on receptive ears. A decade earlier, in 1968, Brezhnev introduced his new dogma: All socialist (read: Moscow-friendly communist) regimes had a responsibility to uphold others, using military force if necessary. The “Brezhnev doctrine” was a response to the “Prague Spring,” a brief period of liberalization under the leadership of Czechoslovakia’s new leader, Alexander Dubček. Even Dubček’s modest steps away from hardcore communism offered reason enough for the Soviets to invade Czechoslovakia and abduct him.

By 1979, Afghanistan, a faltering, once-friendly regime, provided another chance for the USSR to militarily enforce the Brezhnev doctrine. Failing to act, leaders realized, might call into question Soviet willingness to uphold other regimes on its side of the so-called “Iron Curtain,” the physical and ideological border dividing the USSR from the rest of Europe after World War II.

Throughout its history, Russia’s massive territory encompassed a wide swath of national and ethnic groups inhabiting their historical homelands. During the Soviet era, which overlaid a repressive system of centralized power, communist leaders worried about internal challenges erupting in its satellite states—particularly the fast-growing Muslim-majority Central Asian ones.

While propaganda portrayed Soviet life as a happy, multi-ethnic melting pot where different traditions thrived within the context of national unity, the reality for some groups involved purges, deportations and labor camps. To the Soviets, any dissent or shift in alliance from Afghans—even those professing to be communists—posed the risk of sparking similar moves in adjacent states like Uzbekistan, Turkmenistan and Tajikistan, which all shared ethnic identity, religion and history with Afghanistan.

With 20/20 hindsight, it’s easy to conclude that launching an invasion of Afghanistan to prop up an unpopular regime was a foolish, doomed venture. To Soviet leaders in Moscow during the short winter days of December 1979, however, the decision to do just that seemed logical—and inescapable.