When did the Soviet Union start to collapse?

Answer 1

December 25, 1991

source: http://www.soviethistory.org/index.php?action=L2&SubjectID=1991end&Year=1991

"December turned out to be the month in which the fatal blows to the Soviet Union were delivered. On December 1, voters in Ukraine overwhelmingly approved a referendum on independence and by a smaller margin elected Leonid Kravchuk, a former Communist Party boss turned nationalist, as their first president. A week later, at a hunting lodge in Belovezhskaia Pushcha, not far from the Belorussian capital of Minsk, Eltsin, Kravchuk and the Belorussian leader, Stanislav Shushkevich, signed a declaration terminating the Soviet Union and replacing it with the Commonwealth of Independent States. Gorbachev, who had not been consulted or informed beforehand, publicly responded by declaring his "amazement" and urging republic parliaments to discuss the draft Treaty on the Union of Sovereign States on which he had worked tirelessly over the previous months. On December 21, the presidents of all the other republics with the exception of Georgia (already embroiled in civil war) and the three Baltic states, declared their willingness to enter the Commonwealth. Finally, on December 25, Gorbachev announced his acceptance of the dissolution of the Soviet Union and his resignation as its president."

years of economic stagnation left it far behind Western and Asian countries

Answer 2

In December of 1991

Answer 3

The USSR "collapsed" in 1991.

Answer 4

Officially, December 25, 1991.
1991

Answer 5

1991

Answer 6

1989

Answer 7

1985-1991

Answer 8

1989