The US has the highest gun ownership rate in the world. There are literally enough privately held small arms to arm every man, woman, and child in this country. The territory is so large as to be impossible for all but the Chinese [EDIT: and Indians, however no one ever suggests that India will be a potential invader] (with mass conscription) to try to hold with manpower.
The only effective way to invade would involve some combination of nuclear/chemical/biological agents and subterfuge leading to a coup of some sort in multiple regions so that people were peaceful to the invaders.
Given the paranoia present in US culture, and its general xenophobia, this second part seems unlikely.
Instead, you'd end up with a massive, bloody war with low chance of success (at least, the US may be wiped out, but the cost of conquering would decimate the invaders resources).
As for an effective economic coup, that's more feasible. But I'd say the US will do it to itself without needing anyone to push us along. We've been pretty shortsighted about our relationships with other countries for most of our existence, but the past 50 years or so in particular. And we're fucking up our next generation by weakening our education system, driving them into debt for college, and poor mentorship/apprenticeship practices for beginning careers.