Logistics, Logistics, Logistics.....
Lets say you, the invading Force, have Only two ways, by Air or Sea.
By land isn't probable because there's really no way to the North and to the South any gathering of troops that big would be very noticeable, also
very nukeable. Not to mention you would have to get them there also.
Lets see you have to get everything and everyone together, Get them here in one piece, Make sure they take their Objectives, Resupply them, keep their
crap working, Which is a hassle because you probably run metric and the U.S. on on that bassawarkds imperial system, You probably run 220V etc,
Where do you get the fuel for your vehicles etc?
Gas stations won't have enough and will run dry in a week anyway with panic buying etc.
Parts, bring 'em with you.
Ammo, bring it with you, hope you don't run out.
Food: you could scavenge local but since most stores only have a 2 day stock anyway that will be gone by the time you get here.
*note the locals will be Cold, tired, hungry, armed and probably pissed by the time you encounter them.
Red dawn fantasies aside Rednecks won't do well against trained troops.
Trained ex-service members operating in small squads will be a pain in the butt though.
Think, lot of Farmland + lot's Fertilizer = A lot of IEDs.
And these guys learned lessons from Afghanistan and Iraq.
They won't defeat you but they will make your troops wish they had stayed home.
The first troops here will have to be Airborne and they can only carry so much equipment and arms.
Unless they get reinforced soon they will get chewed up and spit out.
That goes for anybody's airborne troops even the U.S.
Read up on the British 1st Airborne division at the battle of Arnhem in WWII
Airborne troops look cool but they are really just the tactical version of a speed bump.
And even if you did use airborne troops how do you get them here?
Chinese air force
Large transport: 80 (A few million divided by 80 = not a lot of leg room )
Special mission: 30
Russian air force Transport Aircraft
620 transport aircraft
Those are rough numbers but they really don't have a lot.
This can be supplemented by Commercial transport as the Russians did in the Invasion of Afghanistan.
But again how to you get your heavy crap over here?
Not feasible unless you sunk all of the U.S. Navy submarines.
Plus you would have a very long and exposed supply chain.
The U.S. subs would run out of ammo before they ran out of targets.
Not to mention U.K. Subs and maybe other allied countries.
So in a nutshell you don't invade the U.S.
You could cripple the U.S. with a Cyber attack, take out U.S Satellites and EMP just for good measure.
And then tactical nukes on U.S. Military targets over seas.
Then rush your troops in and seize strategic areas that you want, Mid east oil Fields, Twain etc.
That creates a political problem, The U.S. and it's allies can't nuke your troops with out damaging the same strategic assets. And they can't
politically justify a Nuke strike on your cities since no U.S. or allied cities were lost.
(But the U.S. has been known to do bat Sh$$T crazy stuff before)
So it comes down to a long protracted conventional war.
But no invasion of the U.S.