Why did the USSR collapse after 72 years?

The Soviet Union, officially established after the Russian Revolution of 1917 and the end of the Civil War in 1922 by Vladimir Lenin, finally came to its collapse in 1991 at the leave of the former Warsaw Pact Eastern European countries. Countless historians and even Russian nationals, have blamed the dissolution of the country upon Gorbachev, the leader who is said to have caused the domino effect of independence within the satellite states through his changing policies. Though …

…to be the lack of an outside antagonistic force. As the US and USSR's relationship improved and the Soviet Union made its resignation in the arms race known, both countries moved into a real détente and no longer were in direct rivalry. Such a lack of an external enemy made it even more difficult for a country with so much diversity to remain one. Considering this, the Soviet empire's collapse was inevitable.