Humans will never acknowledge their common humanity until and unless we face a common enemy that is not human.
As a teenager I read Michael Moorcocks' 1970 novel The Eternal Champion. In it, the protagonist Erekose eventually realises that if he does what he was summoned to do, exterminate the non-human Eldren race, then the currently-united Human race will immediately fall to warring among themselves, and that the only answer which will bring any kind of peace is to exterminate Humanity instead.
In the years since, my studies of history, as well as living through part of it, has done nothing to render Erekoses' reasoning incorrect. Like Tolkiens' Orcs, we fight among ourselves until we find someone else we can hate, all together and all the time, more than we hate ourselves and each other.