Been thinking of this..
There is a few possible solutions.
1. Russia invents some way to protect their homeland from being nuked to the stoneage, and then summons vast forces for an suprice attack. In my eyes, no other faction other than Russia, Or EU/UN could summon such a force in our days.
2. A large submarine attack at the US navy that protects US's costal lines needs to be assebled to clear the way for the landing parties. But still, US subs will be hell due to hunting nuclear subs is nearly impossible.
3. Once that out of the way, the land attacks can begin. this will be rather easy in the beginning if the previous points have gone smoothly, Because of: 1. the unbeatable US army/navy/airfoce is sortof bogged down in the middleast ( or some other provice they attack when the get bored, which incedently is about every 20th year.)
Attacking from either the west or East side would result in the same things.. (yes, attacks could come from canada or mexico.. buuut probably this major military activety.. russians landing troops there would most certainly -not- go unnnoticed)
the first days would go rather smoothly, as the attacker has the element of (hopefully for them) suprice. There will ofc be minor resistance as the home guard manuage to scramble some small forces to put up a little fight, but most likely will be swept away by the sheer size of the attacker that requiered.
Now. Things will go a little slower. As the attacker begins to work its way inland (judgeing by us patrotism nationalism etc etc etc and crazy gun laws) 1. the forces that attacks will get smaller and smaller as they have to leave folks behind in every godforsaken town to keep these rebellious civilians with freakin miniguns in place! And due to the massive size of the country, it takes tiiiime. and that is something you dont have when attacking the US.
Because within a week or so. The forces bogged down in the middleeast would be on its way to save the day.
And even IF! the attacker manuages to somehow destroy this force that comes... the invasion faces some problems.
1. Logistics.
Making war is all about logistics, and invadeing a country potentially full of millions of civilians with loads of weapons, partisan attacks -will- accour, -all the time- at the attackers supply convois.. This never fails to happen in wars, and especially if the invaded countrymen does -not- want the attacker there. And how are you going to keep such a massive attacking force going without fresh gear, food, ammo.. and whatever they might need?
2. its a freaking massive country!!!!
By a week or so, the attacking force will meet an ever growing resistance. Home guard, reserves. Armies have been scrambled to fight off the invader.. the attacks will go slower and slower through desert, steppes, mountains and swamps, cities.. Slower and slower, until the offencive dies..
Conclusion..
this is just a rough sketch of what I would beleave is major points..
But I would say it would be Impossible, or atlest Freakin hard. to invade the US because of the massive size of the country, and the "crazy" people that live there.
There lacks a coutnry that could pull it off. To steamroll through the country, a few could pull that off I beleave. France, germany, england (ive made no research into that, just suggestions of larger miliatary forces.. perhaps all of them together?)..
But without millions of men to station all over the country and strike down at resistance, and military forces that got spared in the initial invasion.. you cant pull it off.
Russia could do it, they have the numbers, but there is no chanse they would do it, they got enough problems as it is allready.
So, I would say.. Maaaaaaybe.. uhm.. yes America could be invanded But you cant hold all of it.
----edit---
Theres one scenario it might work.
Obama turns out to become some evil overlord that rules the US with its rain of terror.
And some country manuages to steamroll over the us, would be greeted as liberators rather than invaders = win?
And to all the fans of the nuke coutner attack..
Lockhead martin and boing has allready invented a way to shoot down long distance missiles as they are still over friendly territory with an sick freaking lazer onboard a boieng 747.