Yeah, you haven't had any Presidents or Senators bombed by disgruntled Irishmen lately, so I'd calm down. Should the USA cross the violence barrier, it will be in a confusing political mess with friendly and opposing zones. The state will use all measures to keep violence contained to "hot" zones, those opposed to the presence of the military, while others will be rewarded handsomely for handing control directly to the Feds. Tit-for-tat violence in these areas between communities, oftentimes false-flagged by the military, will be the norm.
It won't be "foreigners invading" - you likely wouldn't even notice that the guys kicking down your door were Brits or Aussies, you'd be too busy getting shot at. They won't be in Blue Helmets, they'll be using the long-standing cooperation between these forces to pump numbers on the ground - all the guys you'll see standing around will be all-American and with little experience, but the guys who are going to be sniping you in the woods will be experienced soldiers.