I get this question, and have debated it in the past. Basically, it's turning the situation in Iraq around to have the US be the target.
Essentially, we went into Iraq because they have WMDs, and a leader who might use them. During that time, not only did the US have WMDs, but we also had a leader who contemplated using them (Bush, commenting on using nukes to bunker bust). In essence, if a coalition of nations decided we were a danger, they would have just as much right to attempt to do something about it as we had.
If you look at the entire issue from a 50,000 foot/all things being equal perspective.
I'm going to avoid the entire 'might makes right' perspective of the discussion and look at it more realistically. As of right now, there isn't even a collective of nations that could pull this off. It would take the combined might of Russia, China, North Korea, and at least 3 other large militaries to even make it feasible. Just not going to happen.
For this scenario to be realistic, it would have to include a dramatic change in the US, the size of our military, and our leaders ability to control the local population. I say the last part because if the US has sank so far as to make us 'invade-able', there would most likely be a local revolution occurring long before a foreign invasion.
So, if the US had collapsed sufficiently to make a foreign invasion possible, it would really come down to who was invading, and why. While it may look like we liberated Iraq, and just went after their military, a lot of civilians died during the invasion and subsequent occupation. Many of their homes destroyed. Lives demolished in the meantime. The citizenry did not rise up, but there was a substantial insurgency which caused significant issues. I think it's safe to assume that if the situations were reversed, and the US was occupied by 'liberators', there would be a significant local insurgency as well escalating up to established acts of terror to force their hand in vacating the US.
The question being would I join said insurgency? Depends a lot on who is invading, and why.