And what makes you think we want have foreign intervention?
So long as the President of the United States is a real American, he will not invite invaders in. A significant majority of our military is conservative and pro-Constitution. If uninvited foreign intervention shows up, we will have the entire military on our side.
The big mess will most likely be on the purely civilian side, especially in dense cities. That is where most of the dying will happen, and I don’t think even the National Guard will be able to stop it.