UN peacekeepers. out of almost 95K troops 117 of them are from the US for the grand total of .12%. and if you look at the majority of the country's whose member states comprise the largest percentage of troops. they will have ZERO problems firing on Americans.
http://www.un.org/en/peacekeeping/contributors/2013/jan13_1.pdf
there are also dozens of private global security firms out there with tens of thousands of personnel. that would also have zero problems with firing on American's. don't be so naive as to think what the US government wouldn't do to those in the now growing underclass. as US history shows they can and will do anything they want.
lots of people talk a good game until the bullets start buzzing by their heads. and anybody that has been in combat knows exactly what I'm talking about. and all of those cheap ass papermache houses that they have built all over the country don't stop bullets for shit. and don't forget that 50% of the country is obese. wtf are a bunch of fat stupid, lazy, junk food and reality tv watching bastards going to do when the shit hits the fan? die...
so in your scenario the US government invites foreign soldiers on US soil to contain the revolting population? damn, and i thought i was a radical on this issue