OK, so a direct naval invasion is out. The US Navy can defeat any other single navy on earth, and it would take a massive (and massively unlikely) coalition of navies to defeat it.
An air invasion, a la Red Dawn ;), is out of the question for a similar reason. The USAF is the single largest developer of new aircraft technology in the world right now, and even sheer force of numbers (if China, India, and Brazil decided to simultaneously make their whole populations airborne infantry, say) would have a tough time cracking our local airspace.
A land invasion would either fizzle before it got started, due to the efforts of our intelligence services, or make Blitzkrieg-style early gains through sheer surprise before being driven back faster than Napoleon was routed from Russia.
There are, really, only two ways to attack the US from the land: Into the desert southwest from western Mexico (west of the Rio Grande), or into the Rockies and the Great Plains from Canada (British Columbia east to Ontario).
In the first scenario, you end up fighting dedicated tank divisions with massive close-air support in a basically inhospitable climate. Units that advance too quickly will be cut off from the main thrust and pummeled into oblivion, and units that advance too slowly will be halted and pummeled into oblivion. Oh, and beware the locals if you decide to thrust up into the big costal cities (or, if you thought invading Saigon was bad, wait until you have to deal with the LAPD, the California National Guard, the US Army, the Bloods, and the Crips).
In the second scenario, the terrain is either a tank-friendly steppe or a rugged mountain range suitable to long-term guerrilla warfare, with a variation in between. There are a surprising number of military units up here, mostly a remnant of the Cold War-era Strategic Air Command. A B-52 may be old, but the thing can carry an absurd amount of bombs for an essentially indefinite distance. Plus, the locals are well-armed and thinly-spread in the countryside. If you want a partisan waiting to happen, look at the Vietnam-vet hunter with more guns than family members, his wife and three sons, their similar friends and neighbors, and the National Guardsmen who live close by. Plus the Border Patrol.
Speaking of the Strategic Air Command, I think we still have enough nukes to vaporize the first few strata of topsoil and render this planet biologically uninteresting until life has a chance to re-emerge on Mars. And they haven’t been rotting, like the Soviet missiles have. Plus, we have a very definite protocol, multiply-redundant in all the right places, on just what we do if someone decides to nuke us.
So, when attacking the US, getting the first hit in is relatively easy, compared to the task of surviving the fight.