I’m just spit balling here .. General principles for the formation and maintenance of large nations include the technologies for trade transportation, rapid communication, and military risk. All three point in the long run toward greater unification, if not in terms of large governmental entities at least in the sense of very tight alliances between smaller entities. There are many examples for all of those. For trade transportation I like to think of the USA’s Uniform Commercial Code which someone once wrote a book arguing it should have been used as a model for the economic unification of Europe. As technology improved from boats and canals to railroads and then automobiles the benefits to the unification of practices to let trade flourish across State boundaries increased. The basic argument is keep governments separate, but let commercial standardization and freedoms across political boundaries allow economics to work it’s magic. For communications there is now the internet. Back in 1860 the Pony Express was a big deal. For military risk there’s the hydrogen bomb and increasingly the risk of small tactical nuclear weapons being used by terrorists or others. The US as a whole has generally benefited from these trends — Faster communication technology has meant US legal, accounting, and consulting firms have had much better access to foreign markets. Better trade transportation has led to cheaper imports for the US. And military technology actually worked to the advantage of the US in WWII, though Cold War fears were very real. For Germany? Pretty much flip that around. It should not be particularly surprising that the unification of Europe is almost an imperative in the minds of the leadership of Germany. In other regions secession considerations develop because, and I never seem to grow tired of quoting this, “All politics is local” – Tip O’Neil. Ultimately politics is about the neighborhoods people live in. As the diminishing returns to the increasing globalization of trade continue to diminish for many regions, and the perception that large alliances or governments are necessary or even ~effective~ at reducing military or terrorism risk, the pressures against secession weaken and all the usual petty and not so petty frictions of large central governments having some say over people’s lives remain.
Anti-Catholicism . Religion and paranoia about Catholicism helped drive the revolution and secured it the support of America’s Protestant churches. Although the American colonists often preached religious tolerance, in reality they feared Catholics: most colonists belonged, after all, to one of the many Anglican, Lutheran and Presbyterian churches and they feared the impact that the spread of Catholicism and the influence that ‘Popery’ or ‘Papism’ might have on America. The Quebec Act (passed straight after the ‘Intolerable Acts’ of 1774) allowed the French in that particular province to practice the Catholic religion… this fuelled suspicion that the British were ‘soft’ on Catholicism.