I know that in case of emergency there is an OPLAN to divide the country into seven military districts The US divided into 7 districts? That makes for a great deal of "border" to enforce. Although it has some advantages, it also has drawbacks.
What would be the potential for foreign intervention by UN forces or others?
Not much I would think. At least in the sense of making a strategic military difference. It doesn't seem likely that the French or Dutch or Germans would lend many (if any) troops to a UN "peacekeeping" force deployed in the US. However, I can see Obama getting a small number of forces that could be technically seen as UN involvement. Just enough to make the claim that it is a "multinational force" helping restore order. Complete BS, of course.
OTOH, if we envision an international economic crisis / collapse that enabled Obama to justify martial law or such, it seems unlikely that anyone in the EU would be in the condition to loan out troops. The $hit will be hitting the fan everywhere in the west.
Who were our allies in the UN Peacekeeping (what a misnomer) force during the ill fated Somali operation in the 1990s? Remember which army assisted the 10th MTN extract the Rangers & Delta? Pakistan? Of course the UN couldn't make any strategic military difference but that's never the point in those operations. They are never there to actually help in any crisis. They run their own agenda. That agenda is always contrary to American interests. Once we invite the UN into this country we'll never be rid of them. Talk about immigrants doing the jobs Americans refuse! These are the military jobs our own forces would probably always categorically refuse: Interrogation and handling prisoners. Suppose Obama sought help elsewhere than NATO? What about OAS? I'm sure Mexico would love to help regulate lands they have always seen as being historically theirs anyway. Nobody would be in 100% control of the massive amount of territory offered by our country. In such an extreme situation I could see mass relocations under martial law to the vicinity of the bigger cities from the rural areas. Consolidate and control. I mean it's all just conjecture anyway, but I could see it happening. The historical precedent is there in our own history. How many "Peacekeeping" missions have our own forces participated in on foreign soil? What is to say there could never be reciprocity by the UN to come here in a moment of "need?"