Thats a very great question! In my personal opinion, I feel the invention of nuclear weapons is the worst thing to ever happen, because even to this day, we battle the threat of the chances of one country blowing another up with nuclear weapons, which in turn, devastate the Earth, and poses the obvious threat of ending all life itself on Earth. To quote Eisenstein, "I know not what weapons World War three will be fought with, but World War four will be fought with sticks and stones." Probably the only thing worse then that, is the moment in history when humans developed a sense of judgement that allows us to be races, start wars, and use are opinions, and judgement to hurt others. Were the only creatures on Earth who do this to our own species.
Everything. Wars have separated us and allied us, genocide has made us more open minded, and different leaders have made different triumphs and mistakes. Everything from the past has affected something in the future.