Well as primates we haven't even completely mastered the art of a united country. Nationalism is a step shy of world citizenship, which is often thought of as the opposite of nationalism but is, in its purest form, the ultimate evolution of the greater identity concept. Ideally, nationalism would be a friendlier but no less fierce competition between forms of governance, may the best one win, but being what we are, national identity still constantly runs afoul of culture, ethnicity, and politics, making even a united country a very difficult prospect, much less a united world.