The USSR vanished as a legal person on Boxing Day 1991
On 26 December 1991 the Soviet Union formally ceased to exist as a sovereign state. Economic stagnation, arms-race costs, reform chaos, and republic revolts culminated when a failed August coup destroyed Moscow's authority and Gorbachev resigned the previous evening.
Mikhail Gorbachev became General Secretary on 11 March 1985, a few hours after Konstantin Chernenko died at 73; at 54 he was the Politburo's youngest member, and his first aim was to revive a stagnant economy. That July he pushed out his rival Grigory Romanov, and in December he made Boris Yeltsin head of the Moscow party. Liberalisation followed: Andrei Sakharov was allowed back to Moscow from internal exile in December 1986 after a personal phone call, dozens of political prisoners were freed in February 1987, and by late 1988 foreign radio broadcasts were no longer jammed.
Yeltsin soon turned on his patron. Offering his resignation in September 1987, he then attacked the slow pace of reform at an October plenum, and Gorbachev accused him of political immaturity. Samizdat copies of the speech recast Yeltsin as an anti-establishment rebel, and their four-year rivalry helped pull the union apart.
Opened speech also let nationalism flourish. In the Baltics, Helsinki-86 became the first openly anti-communist organisation, and in December 1986 some 300 young Latvians marched through Riga shouting for Soviet Russia to leave. The same month, Kazakh students rioted in Alma-Ata after an ethnic Kazakh party chief was replaced by an outsider from Russia. In 1987 Crimean Tatars demonstrated beside the Kremlin wall for the right to return home, and Armenians marched over Nagorno-Karabakh. Estonia declared sovereignty on 16 November 1988, while the Caucasus slid toward civil war; Lithuania became the first republic to declare outright independence, in March 1990.
The decisive blow was the failed coup of August 1991, when hardliners tried to topple Gorbachev and reverse his reforms, only to wreck Moscow's authority. On 8 December the leaders of Russia, Belarus and Ukraine signed the Belovezha Accords, declaring the union over and founding the Commonwealth of Independent States; Kazakhstan was the last to declare independence, on 16 December. Gorbachev resigned on 25 December, handing the nuclear codes to Yeltsin, and the red flag came down over the Kremlin. The next day the upper chamber of the Supreme Soviet formally dissolved the state, and Russia became its recognised successor.
Source: Dissolution of the Soviet Union
