It appears to me that the United States offers a good illustration of why it is often much better to have political power somewhat restricted in scope – at least when it comes to civil political power. There are actually two reasons I believe this to be the case. The first reason is because I think having a lot of independent militaries provides a good system of checks and balances. Assuming there are 10 nations each of which is committed to freedom and human rights, then if a single nation starts to betray those principles, they will quickly be overwhelmed from rebels from within who can count upon strong military support from the other nine nations. However, in a single nation, corruption can move more invisibly until it has consolidated power, and should a single nation become corrupt and start attacking the rights of its citizens, it will be much harder to put an end to this.
The second reason is one which my wife firmly holds, namely, that people spread across so much area, from so many different cultural backgrounds (because the United States does not even remotely have a monolithic culture), are almost never going to agree on anything. The result is that any democratic centralized authority – perhaps any centralized authority in general – will always be highly dissatisfying to almost everyone. The fact that over half of Republicans are unhappy with their party’s nominee, and that half of democrats are unhappy with their party’s likely nominee, is symptomatic of this phenomenon.
Consider how much better off we might all be, at least in terms of getting the political system which we wish, if we could be split into four or five different nations. A Republican southeast, a neoliberal northwest, a social democrat pacific northwest, perhaps a libertarian New Hampshire, and so on. Granted, the people in the southeast would have horrible health care and be driven into the ground for the benefit of the ultrawealthy. Granted, this would happen to the neoliberal parts too. But then, right now it is happening to everyone.
For this reason, when people speak of wanting to globalize politics, I become quite concerned. The only aspects of politics which I believe should be globalized are human rights standards, which should be enforced everywhere, through imperialistic conquest if necessary. Otherwise, having more localized political power, and having independent political powers that can hold one another in check, is highly desirable.
I look at the recent Brexit referendum through a similar lens.