Events Leading up to the Collapse of the Soviet Union

December 1979: Soviet troops invade Afghanistan to help the country's communist government fight mujahedeen guerrilla forces. (AP)

January 20, 1981: Ronald Reagan is sworn in as U.S. president. During his two terms he implemented a weapons buildup that prompted the Soviet Union to counter with a buildup its economy could not afford. (AP)

March 11, 1985: Mikhail Gorbachev becomes president of the Soviet Union at 54 years old. He introduces glasnost and perestroika in an effort to overcome economic and political problems. (AP)

December 23, 1986: Soviet dissident and nuclear physicist Andrei Sakharov arrives in Moscow after Mr. Gorbachev freed him from internal exile as part of his reform policy. (AP)

February 15, 1989: The last Soviet soldier leaves Afghanistan. (AP)

November 10, 1989: Berliners dancing on top of the Berlin wall, which fell in part due to Mr. Gorbachev's non-interventionist policy. (AP)

May 29, 1990, Boris Yeltsin is appointed chairman of the Russian Soviet Federated Socialist Republic. He becomes its first popularly elected president a year later. (AP)

Mr. Gorbachev survives a coup organized by hard-line communists as an effort to save the Soviet Union. (AP)

December 25, 1991: Mr. Gorbachev resigns as Soviet president, marking an end of the Soviet Union. (AP)