The one event that would unite humanity like no other (even taking advantage of our xenophobia) would be contact with other intelligent life.
We already know at least 5 extinction events on the Earth : https://en.wikipedia.org/wiki/Extinction_event#Major_extinct....
But no proven contact with extraterrestrials.
Probably because life forms on other planets didn't care at their time - didn't pass the Great Filter : https://waitbutwhy.com/2014/05/fermi-paradox.html