Why did the Soviet Union collapse, and could anyone have stopped it?

Several things at once, and historians still debate which mattered most. One account lists chronic economic stagnation, the unsustainable burden of the arms race and foreign conflicts, intense ethnic nationalism in the republics, and the destabilizing effects of Gorbachev's reforms. Gorbachev's own goal, when he became General Secretary, was to revive the stagnating economy, and he realized that this would require reforming underlying political and social structures. But the freedom of speech his reforms brought let nationalist movements and ethnic disputes grow into dominant political movements, and indirectly led to the revolutions of 1989.

The end came fast. Estonia declared sovereignty in November 1988 and Lithuania independence in March 1990. In August 1991 hardliners put Gorbachev under house arrest in Crimea, but the coup leaders neglected to jam foreign news broadcasts, so many Muscovites watched it live on CNN, and Gorbachev followed events on a transistor radio tuned to the BBC World Service. On 8 December 1991 the leaders of Russia, Byelorussia and Ukraine signed the Belovezha Accords declaring that the Soviet Union no longer existed, and on 25 December Gorbachev resigned and handed control of the nuclear launch codes to Boris Yeltsin.

Could it have been stopped? Historians split roughly into two groups. Intentionalists say the collapse was not inevitable and resulted from the decisions of individuals, usually Gorbachev and Yeltsin. Structuralists see deeply rooted structural issues that planted a time bomb.