The big problem with the idea of one-world government or universal empire......
Nations are God's way of weeding out bad economic and political ideas.
For instance, if it had been necessary for the people to rise up and overthrow the CCCP, it never would have happened, the state had an absolute monopoly on weapons. The CCCP fell because the commie bosses themselves were able to get over to Marienplaze and other German and Scandinavian shopping malls and see ordinary European people buying things which they themselves could not own (which their fubar system could not produce or provide).
If on the other hand the whole world had gone commie in the 1930s, there would have been no such basis for comparison and the whole human race might have gone extinct before anybody figured out they had a problem.