The Soviet–Afghan War (1979–1989) was a conflict wherein insurgent groups known collectively as the Mujahideen, as well as smaller Marxist–Leninist–Maoist groups, fought a nine-year guerrilla war against the Democratic Republic of Afghanistan (DRA) and the Soviet Army throughout the 1980s, mostly in the Afghan countryside. The Mujahideen were variously backed primarily by the United States, Pakistan, Iran, Saudi Arabia, China, and the United Kingdom; the conflict was a Cold War-era proxy war. Between 562,000 and 2,000,000 Afghans were killed and millions more fled the country as refugees, mostly to Pakistan and Iran. Between 6.5%–11.5% of Afghanistan's population is estimated to have perished in the conflict. The war caused grave destruction in Afghanistan, and it has also been cited by scholars as a contributing factor to the dissolution of the Soviet Union and the end of the Cold War, in hindsight leaving a mixed legacy to people in both territories.The foundations of the conflict were laid by the Saur Revolution, a 1978 coup wherein Afghanistan's communist party took power, initiating a series of radical modernization and land reforms throughout the country. These reforms were deeply unpopular among the more traditional rural population and established power structures. The repressive nature of the "Democratic Republic", which vigorously suppressed opposition and executed thousands of political prisoners, led to the rise of anti-government armed groups; by April 1979, large parts of the country were in open rebellion.The communist party itself experienced deep internal rivalries between the Khalqists and Parchamites; in September 1979, People's Democratic Party General Secretary Nur Mohammad Taraki was assassinated under orders of the second-in-command, Hafizullah Amin, which soured relations with the Soviet Union. With fears rising that Amin was planning to switch sides to the United States, the Soviet government, under leader Leonid Brezhnev, decided to deploy the 40th Army across the border on 24 December 1979. Arriving in the capital Kabul, they staged a coup (Operation Storm-333), killing General Secretary Amin and installing Soviet loyalist Babrak Karmal from the rival faction Parcham. The Soviet invasion was based on the Brezhnev Doctrine.

In January 1980, foreign ministers from 34 nations of the Organisation of Islamic Cooperation adopted a resolution demanding "the immediate, urgent and unconditional withdrawal of Soviet troops" from Afghanistan. The UN General Assembly passed a resolution protesting the Soviet intervention by a vote of 104 (for) to 18 (against), with 18 abstentions and 12 members of the 152-nation Assembly absent or not participating in the vote; only Soviet allies Angola, East Germany and Vietnam, along with India, supported the intervention. Afghan insurgents began to receive massive amounts of support through aid, finance and military training in neighbouring Pakistan with significant help from the United States and United Kingdom. They were also heavily financed by China and the Arab monarchies in the Persian Gulf. As documented by the National Security Archive, "the Central Intelligence Agency (CIA) played a significant role in asserting U.S. influence in Afghanistan by funding military operations designed to frustrate the Soviet invasion of that country. CIA covert action worked through Pakistani intelligence services to reach Afghan rebel groups." Soviet troops occupied the cities and main arteries of communication, while the Mujahideen waged guerrilla war in small groups operating in the almost 80 percent of the country that was outside government and Soviet control, almost exclusively being the rugged, mountainous terrain of the countryside. The Soviets used their air power to deal harshly with both rebels and civilians, levelling villages to deny safe haven to the Mujahideen, destroying vital irrigation ditches, and laying millions of land mines.The international community imposed numerous sanctions and embargoes against the Soviet Union, and the U.S. led a boycott of the 1980 Summer Olympics held in Moscow. The boycott and sanctions exacerbated Cold War tensions and enraged the Soviet government, which later led a revenge boycott of the 1984 Olympics held in Los Angeles. The Soviets initially planned to secure towns and roads, stabilize the government under new leader Karmal, and withdraw within six months or a year. But they were met with fierce resistance from the guerillas and had difficulties on the harsh cold Afghan terrain, resulting in them being stuck in a bloody war that lasted nine years. By the mid-1980s, the Soviet contingent was increased to 108,800 and fighting increased, but the military and diplomatic cost of the war to the USSR was high. By mid-1987 the Soviet Union, now under reformist leader General Secretary Mikhail Gorbachev, announced it would start withdrawing its forces after meetings with the Afghan government. The final troop withdrawal started on 15 May 1988, and ended on 15 February 1989, leaving the government forces alone in the battle against the insurgents, which continued until 1992, when the former Soviet-backed government collapsed. Due to its length, it has sometimes been referred to as the "Soviet Union's Vietnam War" or the "Bear Trap" by Western media. The Soviets' failure in the war is thought to be a contributing factor to the fall of the Soviet Union. It has left a mixed legacy in the former Soviet Union and in Afghanistan. Additionally, U.S. policies in the war are also thought to have contributed to a "blowback" of unintended consequences against American interests, which led to the United States entering into its own war in Afghanistan in 2001.