December 31: As of this date in 1991 the last official Soviet Union institutions ceased operations and the Soviet Union was effectively dissolved.
The Union of Soviet Socialist Republics (USSR) officially ceased to exist on December 26, 1991 by declaration no. 142-H of the Soviet of the Republics of the Supreme Soviet of the Soviet Union. This declaration acknowledged the independence of the 12 remaining republics of the Soviet Union, and created the Commonwealth of Independent States (CIS).
For most of the Twentieth Century, the activities of the USSR had a huge effect on the United State’s foreign and military policy. Historians have not fully agreed on the dates of the Cold War era, but 1947–1991 is commonly used in textbooks.
The Cold War and its events have left a significant legacy, including major effects on popular culture in the US and Europe. These are especially prevalent in movies and literature around the theme of espionage (such as the internationally successful James Bond film series) and the threat of nuclear warfare that has been used in both dramatic and comedic movies.
Following dissolution of the USST, Russia cut military spending dramatically. Restructuring its economy left millions throughout the former Soviet Union unemployed. These reforms resulted in a recession more severe than the US and Germany had experienced during the Great Depression.
The Republics
Map Key
2. Azerbaijan
3. Belarus
4. Estonia
5. Georgia
6. Kazakhstan
7. Kyrgyzstan
8. Latvia
10. Moldova
11. Russia
12. Tajikistan
13. Turkmenistan
14. Ukraine
15. Uzbekistan
TL;DR Civics 
It is ironic that the actions of hardline members of the Communist Party were ultimately responsible for the downfall of the Union of Soviet Socialist Republics.
Mikhail Gorbachev came to power in 1985 at age 54 after his three elderly predecessors died within a 28 month period. At first cautiously but later more boldly, Gorbachev set out to reform the Soviet economy and political system with his policies which became known as perestroika (restructuring) and glasnost’ (openness).
In addition to his domestic reforms, Gorbachev actively sought better relations with the West. Margaret Thatcher famously declared, “I like Mr. Gorbachev . We can do business together.”. Ronald Reagan who had declared the Soviet Union to be an “evil empire” during his first term went on to negotiate with Gorbachev the first ever treaty between the US and USSR to reduce the number of nuclear weapons.
In 1989 when reform minded forces in East European members of the Warsaw Bloc began to push back against the old school Soviet style regimes in their capitals, Gorbachev’s foreign ministry declared that it would be fine if those countries did things their own ways. This became known as the “Sinatra Doctrine” from the lyrics in the Frank Sinatra song My Way.
The remaining hardliners in the Soviet government and military just couldn’t take any more. In August of 1991 a group of powerful Communist Party insiders detained Gorbachev at his summer home in Crimea and declared to the world that he had resigned due to illness. Of course almost everybody saw through this and regarded it as a coup.
But several days of massive demonstrations in Moscow and the refusal by most of the military to go along with this caused the coup to collapse in days.
But the damage was done. The Communist Party lost credibility because of its cooperation with the coup plotters. Public outrage was widespread.
Various union republics in the USSR decided they no longer wanted to be subject to the whims of Moscow and declared independence. Gradually what was left of the Soviet state disintegrated. The breakup was formalized in late December of 1991.
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