What internal event ended the Soviet Union?

The internal event that significantly contributed to the end of the Soviet Union was the failed coup in August 1991, when hardline Communist Party members attempted to overthrow Mikhail Gorbachev. The coup's failure, largely due to public resistance led by Boris Yeltsin, weakened the power of the central government and accelerated the push for independence among various Soviet republics. This turmoil led to the dissolution of the Soviet Union on December 26, 1991, marking the official end of its existence.