Why did communism in the soviet union collapse in the late 1980s?

consider:

political factors: what did leaders in the soviet union, the united states, and europe do to further the fall of communism?
military factors: what effect did military policies and actions in the united states and in the soviet union have?
economic factors: what effect did the economic situation in the soviet union have?
social/cultural factors: what was the attitude of the people of the soviet bloc countries? what impact did glasnost have? what was the impact of dissidents?