Answer:
1) Mikhail Gorbachev was a young, charismatic leader that pursued more open ideas compared to the aggressive totalitarian hand that most Soviet leaders followed/Glasnost was put into practice and brought open changes to the government that allowed its citizens to criticize their government without fear of imprisonment/Democratization was put into practice and opened the political system that allowed voters to choose their candidates for office.
2) Many nations felt that the Soviet Union was greedy for wanting another satellite state/The Soviet Army had been considered unbeatable, until they lost the war against Afghanistan.
3) China was looking to "step out" of the Soviet Union's shadow and become a communist power/China was spreading their own version of communism through Africa and Asia.
4) The purging and denouncing of the country's memory of Stalin.
Step-by-step explanation:
1) Mikhail Gorbachev promoted a series of reforms on the idea that Soviet system can be reformated to combat corruption and inefficiency in economics and allowing the softening of the most critical aspects of totalitarian control of the State over Soviet society. Economic reform were a disaster and Soviet people responded with the election of Boris Yeltsin as President of Russia.
2) Because Western countries found the Soviet intervention as an imperalistic intervention and did a boycott against the USSR, besides the Afghan war demonstrated the lack of capacity of Soviet millitary on irregular warfare.
3) China refused the De-Stalinization policies promoted by Nikita Khruschev and accused USSR to be 'revisionist' state. In the meantime, China supported guerrillas and communist movements in Asia and Africa.
4) The destalinization was a policy promoted by Nikita Khruschev, succesor of Iosiv Stalin, which consist on the denounce of cult of personality to Stalin.