Final answer:
Mikhail Gorbachev introduced reforms like perestroika and glasnost in an attempt to save the USSR, but they ultimately led to its collapse as Soviet republics sought independence and Boris Yeltsin gained power, leading to the formal dissolution of the USSR in December 1991.
Step-by-step explanation:
Mikhail Gorbachev assumed power in the Soviet Union in 1985 at a time when the nation was under severe financial and political strain. To address these issues, Gorbachev introduced perestroika (economic restructuring) and glasnost (political openness), which were his attempts to save the USSR. However, these reforms brought unintended consequences, leading to increased demands for independence among Soviet republics and the rise of democratic movements. The situation was further destabilized by a failed coup by Communist Party hard-liners in August 1991. Although it failed and Gorbachev was restored to power, his authority was weakened, and the prominence of Boris Yeltsin as the president of the Russian Republic grew rapidly.
Yeltsin and the leaders of Belarus and Ukraine effectively sealed the fate of the USSR when they voted to dissolve the Soviet Union in December 1991. Gorbachev resigned on December 25, 1991, transferring control of the nuclear codes to Yeltsin, who quickly rose to power. The next day, the Soviet Union was officially dissolved, marking the definitive end of the Cold War era.