If I was able to change one of the past events, I would choose to prevent the WWII from breaking out. Of course there were many more horrifying events in the world's history, although this one seems to be a bit personal for me, as my grandparents lost their relatives and got scarred for life because of the war. This way I could not only save millions of lives but also protect my country from destruction. I also believe that maybe the world would be a more peaceful place now, as the Manhattan Project would't had started in the first place and there would be no atomic bombs, or at least there would be no desire for using them...