Without US nuclear disarmament (or US nuclear inventory being neutralized by some technobabble handwavium or something) I don't know if there is a really functional plan for invasion of the US. There were a lot of world leaders and pundits who were seriously afraid of a possible US nuclear response to the 9/11 attacks in the days immediately following them, and it's hard to think of a scenario where an enemy putting an invasion force on US soil not being considered grounds for popping some nukes on opponent's homeland. (Not to mention the possibility of using nukes to destroy convoys and fleets bringing in the main invasion force.)
Even the possibility of a nuke response is a limiter -- we can debate whether the current or previous or hypothetical future presidents have the willingness to escalate things to the nuclear level, but it's always a card in the deck.
So, I'd say the first issue would be that the US would have to voluntarily divest itself of nuclear weapons. Then you'd probably also need the USN and USAF gutted to the point where they can't conventionally contest control of the seas. (Alternately, I suppose you could look at Mexico or Canada being a staging base for an invading force, but there would still be a logistical support coming in from across the Atlantic or Pacific and still the issue of interdiction to address.)
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