Step 1: Understanding the Concept:
The question asks for the year of the formal dissolution of the Union of Soviet Socialist Republics (USSR).
Step 2: Detailed Explanation:
The Soviet Union officially ceased to exist on December 26, 1991. This event was the culmination of a process of internal decay and nationalist movements in its constituent republics, accelerated by the reform policies of Glasnost (openness) and Perestroika (restructuring) introduced by Mikhail Gorbachev starting in 1985. Following a failed coup attempt against Gorbachev in August 1991, the constituent republics began to declare independence, leading to the final dissolution at the end of the year.
Step 3: Final Answer:
The Soviet Union disintegrated in 1991.