Why did Communism fail in the Soviet Union?

On January 1, 1991, the Soviet Union was the largest country in the world, covering some 8,650,000 square miles (22,400,000 square km), nearly one-sixth of Earth’s land surface. Its population numbered more than 290 million, and 100 distinct nationalities lived within its borders. It also boasted an arsenal of tens of thousands of nuclear weapons, and its sphere of influence, exerted through such mechanisms as the Warsaw Pact, extended throughout eastern Europe. Within a year, the Soviet Union had ceased to exist. While it is, for all practical purposes, impossible to pinpoint a single cause for an event as complex and far-reaching as the dissolution of a global superpower, a number of internal and external factors were certainly at play in the collapse of the U.S.S.R.