USSR was a collection of republics, some were willing others were forced in the union. Russia and Ukraine being the largest nations in the USSR. Russia was the powerhouse, and the pivot of the USSR. USSR was ruled by the communists, starting from Lenin, Stalin to Nikita Khrushchev and so forth..
USSR was the antithesis to the USA. Much of the world was divided in two blocs pro USA or NATO or pro USSR, the Warsaw countries. The two had often come close to destroying each other and the world with their powerful nuclear weapons. Hell bent on wiping each other out, the final days of the soviet union forced the two to become sort of allies.
USSR, like an unhappy marriage had run its course and the parties involved wanted a divorce. Russia felt that it carried the weight of the other countries, Ukraine wanted total independence and wanted to ally with the western European nations, Baltic nations wanted freedom and the Muslims nations of central Asia wanted to form their own separate bloc. In the middle of all this was the President of the USSR, Mikhail Gorbachev.