Except that a given nation state surviving one existential crisis may be chance, or even something one could lay to particular events or even individuals.
Surviving four such existential crises (Revolution, change from the Articles of Confederation to the Constitution, 1812-15 war, and 1861-65 war), as the poster outlined, suggests something else, does it not?
No, not really. Indeed, four is a very small number of existential crises by the standards of most nations. For the Habsburg domain, as it existed in various forms, we could undoubtedly find far, far more than four (the Habsburgs fought quite a lot of wars that could have gone much more badly for them in the 19th century alone) and yet that didn't save the Habsburgs. The War of 1812 doesn't even count as an existential crisis; the British didn't especially intend to reconquer the United States so far as I know, merely to stop the USA from fighting them after it declared war on them (whatever one thinks of US justification in doing so, on which I will not comment here lest I go too far off-topic again, it is undeniable that the USA chose to go to war whereas the UK did not deliberately start it), preserve Canada-to-be, claim some border territories (Maine, perhaps?) and perhaps constrain US expansionism at the expense of the Native Americans (if one holds the belief that the British ever truly intended to remain loyal to the Indian Confederacy rather than using Tecumseh as a useful patsy). That's not an existential crisis.
The point of all this is not to argue that the USA was an especially unstable, but, rather, to argue that surviving multiple existential crises is not a particular indicator of an ability to
always survive future crises.
Are you genuinely contending that the USA could
never have fallen with
any PoD, even as far back as 1776? The most obvious answer is British victory in the American Revolutionary War, which was far from impossible (there are multiple battles where, had they gone differently in manners that were far from impossible, the Americans would have lost the war) and was, arguably, at least as probable as what actually happened IOTL; in the event of a British victory in that war, Great Britain's American colonies might well have achieved independence in a later war (indeed I'd argue that later American independence is very likely indeed in that scenario) but any independent American nation(s) in that scenario would be very unlikely to be the same as OTL's United States.
In fact, one might event make the judgment it reveals a truism, as in the Americas (including the United States, of course) occupy a fairly unique geographic niche - continent-wide (mostly) temperate bands, close enough to Europe and Africa to benefit immensely from the eastern hemisphere's population "sending" ability, and yet far enough away to be safe from the political and military power of Europe.
Gran Colombia was also in the Americas. Geography doesn't make it impossible for a polity to collapse. If (e.g.) the various northeastern US states don't give up their claims on one another and the federal government stays exceedingly weak due to a retention of the Articles of Confederation, with such things as the federal government never assuming state debts, I am yet to be convinced that the USA will stay together just because it did IOTL; 'general resilience' is rather vague.
Which is why, for example, European efforts to control the Western Hemisphere, once the "locals" were more or less autarkies, all failed - continually and repeatedly.
IOTL, yes. I daresay the existence and the power of the United States had quite a lot to do with that. And the fact that France acted exceedingly foolishly and unsubtly in Mexico doesn't mean that all hypothetical European powers would always act in such a manner in all possible ATLs.
If (for example) the USA hadn't cared at all about Venezuela and a major European power had intervened there, do you imagine that the Venezuelans would have won? If some European power rather than the USA had exerted imperialism against Colombia and chosen to break off Panama, do you imagine that the Colombians would have won? Perhaps in the long term, a few decades later, and perhaps the European power would withraw later of its own accord if it had some other reason to do so; but that applies to US interventions just as to European ones. But there is no never-failing axiom which says that nations in the Americas must always defeat European powers trying to control them. There is a general advantage of being an ocean away, I do not and will not dispute that, but I remain unconvinced that that advantage is an unbeatable one in all possible ATLs.
Also, if you hold to the position that all those efforts failed, I'd be interested to see how you would describe the period of 'informal empire'.
It also suggests why, despite the above, the Western Hemisphere republics generally remained concerned enough regarding the European threat to manage to hang together
As the USA and Mexico hung together? As the various Spanish-speaking states of the Americas hung together? As the constituent parts of Gran Colombia hung together?
And lest I be tarred with one particularly dark brush, it is possible to argue that the USA was not historically invincible without being a neo-Confederate apologist and without holding any personal bad feelings towards the USA, just as it is possible to argue (on ground more familiar to me) that Prussia could have failed to unify Germany without holding any personal bad feelings towards either Prussia or Germany.
