In April 1978, the communist People's Democratic Party of Afghanistan (PDPA) seized power in Afghanistan in the Saur Revolution. Within months, opponents of the communist government launched an uprising in eastern Afghanistan that quickly expanded into a civil war waged by guerrilla mujahideen against government forces countrywide. The Islamic Unity of Afghanistan Mujahideen insurgents received military training and weapons in neighboring Pakistan and China, while the Soviet Union sent thousands of military advisers to support the PDPA government. Meanwhile, increasing friction between the competing factions of the PDPA—the dominant Khalq and the more moderate Parcham—resulted in the dismissal of Parchami cabinet members and the arrest of Parchami military officers under the pretext of a Parchami coup. By mid-1979, the United States had started a covert program to assist the mujahideen.

In September 1979, Khalqist President Nur Muhammad Taraki was assassinated in a coup within the PDPA orchestrated by fellow Khalq member Hafizullah Amin, who assumed the presidency. Distrusted by the Soviets, Amin was assassinated by Soviet special forces during Operation Storm-333 in December 1979. A Soviet-organized government, led by Parcham's Babrak Karmal but inclusive of anti-Amin Khalqis, filled the vacuum and carried out a purge of Amin supporters. Soviet troops were deployed to stabilize Afghanistan under Karmal in more substantial numbers, although the Soviet government did not expect to do most of the fighting in Afghanistan. As a result, however, the Soviets were now directly involved in what had been a domestic war in Afghanistan.

Carter responded to the Soviet intervention by withdrawing the SALT II treaty from ratification, imposing embargoes on grain and technology shipments to the USSR, and demanding a significant increase in military spending, and further announced that the United States would boycott the 1980 Summer Olympics in Moscow. He described the Soviet incursion as "the most serious threat to the peace since the Second World War".