What prompted the collapse of the Soviet Union?

The cold war was one of the main reasons because the government was overspending and their economy started to fail then there was a coup (sudden overthrow of the government) and the soviet union split into smaller countries

Durning the leadership of Gorbachev because of Perestroika and Glasnost. These reforms gave more power to the individual states in the union and allowed for a Communist-Capitalist hybrid. The states now enjoying more freedoms began to have nationalist movements, gaining independence. Some said Gorbachev was too harsh, while other viewed him as not harsh enough.