It's an interesting question, because the actual detonation was just going through the motions of something that had been built up to for years. And if no nuclear holocaust ever happens, then nuclear weapons will seem less important. But obviously, the Sword of Damocles is very important even if it doesn't fall on you.
Maybe the first atomic bomb detonation?
I'd second that. It changed everything.
It's an interesting question, because the actual detonation was just going through the motions of something that had been built up to for years. And if no nuclear holocaust ever happens, then nuclear weapons will seem less important. But obviously, the Sword of Damocles is very important even if it doesn't fall on you.