In 1945 the United States became the only nation to ever launch a nuclear strike on another country. In a two moment's flashes we killed hundreds of thousands of individuals - the vast majority being innocent civilians playing no direct role in the war whatsoever. But that act of arguable amorality not only immediately ended a world war, but has since led to a great fear of war among any nation with these weapons which has led to an unprecedented period of relative world peace. This [1] great video shows the borders of Europe throughout time. The sudden lapse in that constant warfare and transition, the one we now still live in, that occurs just shortly after the more widespread development of nuclear weapons is striking.
But nuclear weapons will not remain the ultimate weapon and deterrent forever. And whichever nation is able to develop appropriate defenses against nuclear weapons and offenses beyond nuclear, will be the same nation that decides the direction of our species moving onward into the future. Again, it's a lowest common denominator problem - but it's one that's ultimately unavoidable by any means other than by ensuring that you yourself are always the most militarily capable nation. And ideally multiple nations will develop these weapons in unison. I'm not particularly fond of e.g. China having unopposed reign throughout the world, but neither am I fond of the idea of an unopposed reign of e.g. the United States. I think we find the best outcomes when we have multiple balanced powers, and in this regard it is our responsibility to never fall behind.