I would say that if you want an event then it would have to be WW2, the holocaust and associated tragedies.

If you want a concept I would have to say intolerance.

What causes war? Intolerance. Racism? Intolerance. Terrorism? Intolerance. Religious genocide and warfare? Intolerance.

If human beings could just learn to put up with each other and damn well work together then a hell of a lot of problems would disappear. And then we could band together and solve the rest of them (like disease, famine, poverty and global climate change)