The US is the exception that confirms the rule, but mainly because it managed to basically conquer/occupy a continent-size territory, from ocean to ocean.
Russia did try the same thing, that's why they managed to reach the Pacific pretty early on, comparatively speaking, i.e. in the late 1600s - early 1700s. It was on the Western side and partially on the Southern side where that wasn't achievable.
Late in her life Catherine II (or people close to her, anyway) did dream about having 6 Russian capitals scattered around the Euroasian continent, including in Berlin and Vienna, but that was not achievable (and still isn't, I'd say). Stalin immediately post-WW2 came pretty close to it, though. On the Southern side there were the issues with the Caucasus, Iran/Persia and present-day Afghanistan, which had conflicting results and outcomes for Russia/USSR.
The US is hardly an exception. It conquered the west, invaded Mexico and even tried to invade Canada once precisely because of this geographical determinism.
Its later stability is a testament to just how geographically secure its borders are.
If, in an alternate timeline, America was a weakened ex super power and Canada joined the Warsaw Pact I imagine it would take all of 5 minutes for the tanks to roll into Vancouver.
The US became the US after all the events that you mentioned, I’d say that after 1865.
As I said, Russia has been on the same quest of finding insurmountable natural borders that would have defined for it for good, but geography doesn’t help it. And during that quest it also tries to re-define itself in order to reach its final form, so to speak, again, much as red US had done.
Main stream writing on the topic is motivated to focus on the GEO in geopolitics, because talking about geographic features and demographic pyramids is cleaner than wading into politics, especially when intermingled regional foreign politics. But the POLITCS does a lot of heavy lifting, whereby politics influence things like tech innovation/adoption that can tame geography, i.e. restitive frontier regions being incorporated by rail infra, mass public works or inventions like AC/desalination make improving previously inhospital/unpassable geography. Or in defense, gunpowder/mechanization mitigated overwhelming advantage of static geo based defense, and similarly going forward, long range precision strikes (conventional ICBMs / advanced rocketry) will reduce all global favourable geo conditions to extreme vunerability. How will US preeminence fare if she has no impenetrable homeland to operate from etc.
It is very good at confirming pretty much anything but is awful at predicting future events. Pseudo-scientific tosh akin to Freud.