In the U.S. the Reagan Administration increased the budget for the military and presented the possibility that it would implement a Star Wars antiballistic missile system. To maintain a parity with the U.S. under those developments would have required an even larger share of industrial ouput going to the military. The planners and decision-makers had to face the fact that it was economically impossible for the Soviet Union to increase the share of its output going to the military.
The Soviet authorities then ended the arms race and called off the Cold War. When the justification of an external threat was removed there was no reason for the Russian public to toleratel the totalitarian regime and the political system fell apart.