You will never unite mankind as a whole. People hate each other for their differences, and this will never, ever stop, not in a billion years. There are a few people who *think* they would like a giant freakish melting pot of cultures, but I think America has already proven this to ultimately be a colossal failure. Sure, we have a bunch of people living together, but America is far, far from unification of any sort. All it is is a bunch of separate cultures keeping to themselves as much as possible and working the political system to try and help their own interests, completely ignoring the interests of everyone else. It's pointless, stupid, and will more than likely lead to the fall of the American empire before much longer.
I'm not saying it's impossible for people to get along. What I'm saying is you can never get groups of people in the numbers that "nation" implies to rally around the same causes, and there will always and forever be division because of that. Even if you get smaller groups together in harmony (except that as noted, there's nothing resembling harmony even between such small groups as it stands today), attempting to unite the world is a pipe dream of the most epic proportions.