The event associated with the dissolution of the Soviet Union in 1991 is the formal declaration of independence by Ukraine on December 1, 1991. Following a period of political upheaval and reforms known as glasnost and perestroika under Mikhail Gorbachev, the Soviet Union faced increasing nationalist movements in its republics. Ukraine's move for independence, supported by a popular referendum, signaled a definitive shift that contributed to the collapse of the Soviet Union. Subsequently, on December 25, 1991, Gorbachev resigned, and the Soviet Union officially dissolved, leading to the emergence of independent states and a new geopolitical landscape.