How did realpoliitk contribute to the end of the Cold War?
It is not always easy to tell when realpolitik is being practiced because you have to know the motives behind a country's actions. However, we can speculate about at least two episodes that show realpolitik helping to end the Cold War.
The first was the opening of diplomatic relations between China and the US. Ideologically, there was no reason for this. But in terms of realpolitik, this was a good move because it put pressure on the Soviet Union. This helped to end the Cold War (though not right away) by reducing the cohesion of the communist world.
The second was Ronald Reagan's push for a new arms race. He felt that this would weaken the Soviets. If Reagan did this out of hatred for the communist system, it is not realpolitik. However, if he did it because he thought it would increase US power and security, it is realpolitik. Whatever the motive, this move helped end the Cold War by hurting the Soviet economy as it tried to keep up in the arms race.