bcaiko
Emperor
Bingo.
I am a little surprised that conquering 1 city-state got the OP dogpiled like that. The last game where I had the entire world declare war on me was because I attacked a CS early in the game, Germany asked me to end the war and I did (just needed a worker), then later annexed the city and somehow "broke my promise" to Germany. This was a huge diplomatic hit and caused a cascade of denouncements, including from friends. Add in that I had already attacked my too-close-for-comfort neighbors, and...world war.
But I've played games where nobody really cared that I took a CS. Just need to do it early in the game.