'X' was the general secretary of the Communist Party of the Soviet Union from 1985 to 1991 and president of the Soviet Union in 1990–91. His efforts to democratise his country's political system and decentralise its economy led to the downfall of communism and the breakup of the Soviet Union in 1991. In part, because he ended the Soviet Union's postwar domination of eastern Europe, 'X' was awarded the Nobel Prize for Peace in 1990. Identify 'X'.