It's pretty obvious that tragedies, no matter how awful, don't do it. Or if they do it only lasts for a day at most before everyone starts finger pointing and looking for places to throw blame.
But do you think there's anything that could ever possibly bring us all together? Outside of some science fiction stuff like an alien invasion, I'm starting to think there isn't. And even then I bet there would be groups of people who would quickly side with them.