A bit of American and World History.
America started as individual colonies, belonging to different European countries. These colonies formed their own constitutions and governments. They were truly individual sovereign states under the direct control of foreign powers.
There was a strong threat from the European countries to continue to dominate the 'colonies', and so they decided that perhaps forming into a mutual defence pact would be to their advantage. An individual colony could not survive, It had neither the military nor the economic strength to survive for very long. But together under a mutual economy and defence they had a chance. However, none of them wanted to give up either their constitutions, or their sovereignty. So, they all got together to hammer out some form of constitution much like the European Union today - sovereign states united under a common currency and defence strategy. Each would have its own legislature, court system, justice system, and internal political structure and political parties. (Foreign relations and trade was a very big discussion point). The federal oversight government had very strictly limited powers. The POTUS system was primarily ceremonial. It was envisioned that America would never have a national army, but defense would be in the hands of individual state militias (this continued even up until the Civil War. The 'cavalry' was more like a national internal police force).
This is the form that many constitutionalists want to return to, when they say they want to interpret the constitution as written. If they (the GOP) win, America will return to a republic of sovereign states much like the EU, and the federalist powers would be drastically reduced.
However, over time the original constitution has been so badgered that the federal government and federal court has become supreme, and the individual sovereign states have given up more and more of their sovereignty, much like the EU is going through now.
During the Civil War, the South tried to flex its constitutional right and power for self-determination, but the North pushed its federalist agenda, and emphasized the 'one nation, indivisible' line of the constitution. This battle is still going on.
Other nations, like Canada and Australia, have formed different variations on the 'sovereign states' concept, with a constitutionally mandated division of power between the federal and the provincial governments. Canada is somewhat unique, in that it has mandated that each of the founding cultures be represented federally, across all Canada, instead of being ghettoized into specific regional areas.
However, over all of world history, it has been repeated over and over, that once an empire gets too big, it fails under its own bureaucracy. The bigger it gets, the more power has to be divested into local regional bureaucrats, who themselves become just as rich and powerful as the central government. This is how Rome fell. Too many strong generals. Strong generals were needed to protect the state against strong enemies, but the bigger the state, the more strong generals were needed. They began to compete against each other.
This is happening in America. California is the fifth largest 'nation' by GDP, equivalent to France, and this has given them much economic power. They have a lot of clout that enables them to get their own way, and paradoxically the GOP agenda to diffuse federalist power only feeds into their independence.
There is a natural limit on the size a country can be before internal division and regional interests conspire to cause its demise. Only a nation that consists of sovereign states, with their own internal legislative agenda, but united by a common economic and social responsibility agenda, has any real hope of tens-of-centuries-long survival. Sort of like a UN that has policing teeth to enforce a universal standard of human rights, with individual small sovereign states determining their own futures. It would need to have universal rights to unrestricted immigration and emigration, a universal exchange currency, and a strict policy on upholding basic human rights and equality.