I mean 2 parts. 1: Ukraine is fighting off a bigger countries tanks with Molotovs.
2: Logistics, supplies, etc is a HELL of a fight in two directions and the US has a hell of a home field advantage. If someone (I'm going to say from the Asian continent for this argument) any supplies they want has to cross the ocean. Then right now unless we do something to massively piss them off, we've got allies to the north and south so no country is giving a foothold next door (This is why South Korea and Japan being allies is important to the US, and why China really likes North Korea being there. So far the only foothold the biggest enemies to the US can get is Cuba... which right now involves circling the globe the even longer way).
But now our hypothetical military force has beaten back the US navy which is filling the oceans with all sorts of attacks, they get to the shores where they contend with the US Coast Guard, the US's second navy. That gets beaten back, and now the land war has started. Lets say they take LA and that's where the invasion is starting.
"Defeating" the US Military is truly a defeat of the US because now it's a home front war. You'll have the US military fighting on US soil, which I think the last time that happened was the civil war (correct me if I'm wrong), the Reserves are getting called in. Then even before you get to the "random guys with guns" the actual US militia gets mobilized where the National Guards of each state is called in. And you better believe they'll definitely take their gun nutty neighbors in this because the national guard works day to day in civilian world.
So now we've gotten to logistics. The US likes to beat its chest when it comes to military, but the true might of the US in history has been logistics of "We need x here today" and we can ship troops to the other side of the globe faster than Amazon can deliver a package to a doorstep in country. The US has a robust interstate system, designed after the autobahn of Germany for the same purpose, moving hardware. So whatever military is fighting now has to contend with whatever front line is existing getting supplied by the factories in the middle of the country with semis running supplies daily as well as military hardware from the side of the country not getting attacked at the moment (I live near multiple military bases that's as far from any border as one can logically get, where there's tank divisions just waiting).
But we're calling a defeat scenario for the US, so the hypothetical beats back the US military, who's probably tearing apart the infrastructure as it backs up if it's smart and plays like the Russians do. Granted the US doesn't have a Russian Winter, but if you're coming from the West you have the Rockies, coming from the East you have the Appalachians. Mountain ranges that makes mobilization difficult if the infrastructure is fucked, but the infrastructure is fine on the other side.
I legit cannot imagine a country, even the US, with the infrastructure to break through that wall scenario delivering hardware across the world.
But hey, we don't have to talk a complete invasion of the US. Just some area. Remember, many US states are as big as countries, especially the western ones. So an invasion happens, now you're dealing with large swaths of territory. Russia had trouble with the Finns in the Winter War, with Russia being right next door because the Finns didn't just up and fight the major people. No they'd let tank battalions pass, then when the logistics crew following the battalions showed up they'd get sniped. Or the US tried to fight in Vietnam and were beaten back by civilians. Or occupied Afghanistan and ultimately the Taliban managed to regain control. Occupation is REALLY hard because even for a small territory you have to have a large soldier to civilian ratio for those "military grade" weapons and tactics to beat civilians (the difference between a "military grade" AR-15 and a civilian one is the ability to turn it to fully automatic. That is something that modifications exist to do.) And if an invasion is coming I guarantee you every Scheels, Bass Pro, Cabelas, Academy Sports, and every mom and pop gun store will be having a fire sale on ammunition.
TLDR: It would be a logistical hurdle to even shut down US military bases before reaching one of the mountain ranges, while the US would be sitting in its logistics hub. Invading forces only really truly succeed historically long term when the civilians are on the side of the invaders because occupying long term is really hard, and I think you're underestimating the military logistics of a country that has ammunition vending machines.
US Here, I'd like to but not having the skill sets right now makes for difficult to move to another country.
Which leads to another problem, if I do I leave my entire social network behind to a culture that I don't know and trying to live there. While I'm not against that, I realize that can be VERY isolating so not sure where the place I'd want to go right now.