The advent of nuclear weapons, it can be argued, made it so that there would no longer be any major power wars. Because all of the major powers had nuclear weapons, they were not able to fight one another the way that they had in, for example, WWI and WWII.
Before nuclear weapons, it was not that "dangerous" for major powers to fight. A country like Germany did not risk its very existence by going to war in 1939. But if the US and the USSR, for example, had gone to war in the 1960s, they would have been risking their existences. Both countries could have utterly destroyed the other with their nuclear weapons.
For this reason, the advent of nuclear weapons changed the kinds of war that were possible in the years after 1945.