The Soviet government deployed troops in Afghanistan, starting the Soviet–Afghan War.

The Soviet–Afghan War took place in the Democratic Republic of Afghanistan from December 1979 to February 1989. Marking the beginning of the 46-year-long Afghan conflict, it saw the Soviet Union and the Afghan military fight against the rebelling Afghan mujahideen, aided by Pakistan. While they were backed by various countries and organizations, the majority of the mujahideen's support came from Pakistan, the United States, the United Kingdom, China, Iran, and the Arab states of the Persian Gulf, in addition to a large influx of foreign fighters known as the Afghan Arabs. American and British involvement on the side of the mujahideen escalated the Cold War, ending a short period of relaxed Soviet Union–United States relations. Combat took place throughout the 1980s, mostly in the Afghan countryside, as most of the country's cities remained under Soviet control. The conflict resulted in the deaths of one to three million Afghans, while millions more fled from the country as refugees; most externally displaced Afghans sought refuge in Pakistan and in Iran. Between 6.5 and 11.5% of Afghanistan's erstwhile population of 13.5 million people is estimated to have been killed over the course of the Soviet–Afghan War. The decade-long confrontation between the mujahideen and the Soviet and Afghan militaries inflicted grave destruction throughout Afghanistan and has also been cited by scholars as a significant factor that contributed to the dissolution of the Soviet Union in 1991; it is for this reason that the conflict is sometimes referred to as "the Soviet Union's Vietnam" in retrospective analyses.