I was ahead in culture, techs, money, everything (except a little behind in military units- not quality, but quantity). I even had two-thirds of the
Unity finished. Then all of a sudden, I hear that QEI of England is building the United Nations. I think, "I will just sabotage THAT when she is almost finished with it".
In the meantime, I decided to build the Manhattan Project and drop my prototype tactical A-bomb on a neighboring Iroquois city. Boy did that tick everyone off! But actually, my closest allies were still on fairly good terms with me. The AI civs actually reacted quite realistically to my A-bomb drop: some were ok with it (allies), but my main competitor (England) declared war outright.
Needless to say, I did not win by UN victory.