According to the media, the world is already against us, right? The scenario you present is frighteningly plausible, but consider:
The UN, if it sends foreign troops in to take our guns, will start a war they cannot win - it will indeed be tragic and, with their body armour and equipment, intially lopsided in casualty ratio (until we start using the equipment we have taken off their dead bodies against them), but they cannot subdue us - we WILL triumph. Not a pretty picture, for sure, but the "militia of flight 93" proved the metal of the "average American", and I'm confident that this spirit will, ultimately, bring us out on top. "We'll put a boot in [their] a$$, that's the American way!" (with all due apologies to Toby Keith).
I just hope and pray it never comes to the point you and I are discussing!
And considering how wishy-washy the UN has been of late, it may not even come to that. Doubt they would have the stomach to deal with a few guerilla-style killings.