What was a primary cause of the collapse of the soviet union in the late 20th
century?

Gorbachev's decision to allow elections with a multi-party system and create a presidency for the Soviet Union began a slow process of democratization that eventually destabilized Communist control and contributed to the collapse of the Soviet Union.

the correct answer is:   the domino theory states that if one country in a region came under the influence of communism, then the surrounding countries would follow in a domino effect.