1980 Soviet occupation of Afghanistan (State Department Historical e-summary)

Finally, on Christmas Eve 1979, the invasion began. Soviet troops killed Hafizullah Amin and installed Babrak Karmal as the Soviet’s puppet head of government.
Although the Carter administration had closely watched this buildup from the outset, its reaction following the invasion revealed that, until the end, it clung to the hope that the Soviets would not invade, based on the unjustified assumption that Moscow would conclude that the costs of invasion were too high. In response, Carter wrote a sharply-worded letter to Brezhnev denouncing Soviet aggression, and during his State of the Union address he announced his own doctrine vowing to protect Middle Eastern oil supplies from encroaching Soviet power. The administration also enacted economic sanctions and trade embargoes against the Soviet Union, called for a boycott of the 1980 Moscow Olympics, and stepped up its aid to the Afghan insurgents.
https://history.state.gov/milestones/1977-1980/soviet-invasion-afghanistan