"The dissolution of the Soviet Union (1988–1991) was the process of internal political, economical and ethnical disintegration within the USSR as an untoward result of General Secretary Mikhail Gorbachev's effort of political and economic reform of the Soviet authoritarian system and declining planned economy, which resulted in the end of its existence as a sovereign state. After a 1991 military coup attempt in Moscow led to a complete disintegration of the country, Gorbachev had to first retire as leader of the Communist Party since it practically ceased to work, then to resign his office as president and what was left of the parliament to formally acknowledge the Union's collapse as a fait accompli.

The process began with growing unrest in the Union's various constituent national republics developing into an incessant political and legislative conflict between them and the central government. Estonia was the first Soviet republic to declare state sovereignty inside the Union in 1988. Lithuania was the first republic to officially break away from the USSR and restore independence in the Act of March 11, 1990 (not counting the autonomy of Nakhchivan, which had declared independence from both the USSR and the Azerbaijan SSR a few weeks earlier, later rejoining Azerbaijan).

The failure of the coup, when the KGB and military elites tried to overthrow Gorbachev and stop the failing reforms, led to most republics proclaiming independence in the following days and months. The secession of the Baltic states was recognized in September 1991. The Belovezha Accords were arbitrarily signed on December 8 by President Boris Yeltsin of the Russian SFSR, President Kravchuk of Ukraine, and Chairman Shushkevich of Belarus, recognising each other's independence and creating the Commonwealth of Independent States (CIS) instead of the Soviet Union. Kazakhstan was the last nation to leave the Union, proclaiming independence on December 16. All the republics, with the exception of Georgia and the Baltics, joined the CIS on December 21, signing the Alma-Ata Protocol.

On December 25, Gorbachev resigned and turned over his presidential powers—including control of the nuclear launch codes—to Yeltsin, now president of the Russian Federation. That evening at 7:32 p.m., the Soviet red banner was lowered from the Kremlin for the last time and replaced with the Russian tricolour flag. The following day, the Declaration 142-Н of the Supreme Soviet's upper chamber, the Soviet of the Republics, recognised self-governing independence for the former Soviet republics, formally dissolving the Union. Both the Revolutions of 1989 in the Eastern Bloc and the dissolution of the Soviet Union marked the end of the Cold War.

In the aftermath of the Cold War, several of the former Soviet republics have retained close links with Russia and formed multilateral organizations such as CSTO, the CIS, the Eurasian Economic Community, the Union State, the Eurasian Customs Union, and the Eurasian Economic Union, for economic and military cooperation. On the other hand, the Baltic states and many of the former Eastern Bloc states became part of the European Union and joined NATO, while some of the other former Soviet republics like Ukraine, Georgia and Moldova have been publicly expressing interest in going the same path since the 1990s."

       (from Wikipedia)

The Soviet Collapse: Contradictions and Neo-modernisation (Sakwa)