How is mankind ever going to work together to better ourselves and maybe colonise other planets?

lets face it almost all humans only work together if we have to, so unless were pushed into cooperation by natural disasters (quite likely with global warming) nuclear war (possible) or alien invasion (anything could happen), we're never going to be just 1 nation as there is to much difference of opinion there would just be constant revolts. However the budget and collective needed for things such as space exploration and other progressions in technologies, are more than any one nation could field even the US, so without total unity away from political boundaries and shared resources how can we better ourselves?