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1) They don't want to anger US allies.
2) They don't want a nuclear device detonated in their capital city.


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No, those are good answers but they do not completely eliminate any smidgen of a possibility of an invasion led onto US soil. They act as strong deterrents to an invasion, but do not outright eliminate any possibility of it even happening.

Im not trying to be a dick here, but if you or anyone wants to take a stab at why the US cannot possibly be invaded right now, how it is not even mathematically or even theoretically possible to invade the US under any circumstances whatsoever, please do so.

The sustenance of these two words ties greatly into our massive defense budget. These two words require a great deal more than "far less than 10% of our current military".