In conjunction with the barbarian conflict, I could foresee the native resistance arising in connection with a simultaneous political initiative to integrate the US with a larger entity, probably either pan-American or trans-Atlantic.
When enough barbarians are present in enough states, the combined socialist/barbarian/foolish&gullible vote will be enough to keep in office politicians who favor the political integration project which, like the EU, would be done largely through the easier treaty process than by constitutional amendment or referendum.
Those unwilling to submerge the US into a larger entity could very well need to be suppressed by a combined transnational force reflecting the larger political entity.
Hmmm.
The resistance forces would be small as long as the majority of the US population was kept tranquilized by “bread and circuses.” Could be done.