The disintegration of the USSR was a complex and multifaceted process that occurred over several years, but it can be attributed to several immediate causes.

Economic Crisis
One of the primary causes was the economic crisis that the Soviet Union faced in the 1980s. The Soviet economy was heavily dependent on oil exports, and when oil prices fell in the 1980s, the Soviet Union faced a severe economic crisis. The Soviet economy was also inefficient and technologically backward, which made it difficult to compete with other global economies.

Political Reforms
Another immediate cause was the political reforms instituted by Soviet leader Mikhail Gorbachev. Gorbachev introduced a series of reforms aimed at modernizing the Soviet economy and political system, but these reforms had unintended consequences. The reforms led to increased political freedoms and the emergence of opposition movements, which ultimately contributed to the disintegration of the Soviet Union.

Nationalism
Nationalism was another significant factor contributing to the disintegration of the USSR. As the Soviet Union began to experience economic and political turmoil, nationalist movements emerged in various republics of the Soviet Union. These movements sought greater autonomy or independence, which further weakened the Soviet Union.

Ethnic Tensions
Ethnic tensions also played a role in the disintegration of the USSR. The Soviet Union was home to many different ethnic groups, and tensions between these groups increased as the Soviet Union began to unravel. In some cases, these tensions led to violent conflicts, which further contributed to the disintegration of the Soviet Union.

Conclusion
Overall, the disintegration of the USSR was caused by a combination of economic, political, nationalist, and ethnic factors. The collapse of the Soviet Union was a complex and multifaceted process that occurred over several years, and it had significant implications for global politics and international relations.