With the country in a state of near collapse, Gorbachev's vision of a renewed union effectively received a fatal blow by a Ukrainian referendum on 1 December, where the Ukrainian people overwhelmingly voted for independence. Ukraine had been the second most powerful republic in the Soviet Union after Russia, and its secession ended any realistic chance of the Soviet Union staying united even on a limited scale. The presidents of Russia, Ukraine and Belarus met in Belovezh Forest, near Brest, Belarus, on 8 December and signed the Belavezha Accords, which declared the Soviet Union had ceased to exist and formed the Commonwealth of Independent States as its successor. Gorbachev initially denounced this move as illegal. Nonetheless, there was no longer any doubt that the Soviet Union, in the words of the Accords' preamble, no longer existed "as a subject of international law or geopolitical reality."
Source: Wikipedia