If we are ever in a situation where the citizens are facing a firefight with armed forces of the government, once they show up with Apache gunships, that will pretty much end any fight.
I also do not think that will ever happen.
I think DHS must be contemplating having to support military action against an invading force. If that is the case (and I have to say I am sure they are planning for that exact set of contingencies) I will be very happy to see the MRAPs roll out.
I think we could possibly be facing off against the Russians, Chinese, North Koreas, and enemies in the Middle East and Africa, either singular or combined forces, also either in a WWIII scenario, or sleeper cell infiltrators carrying out a massive coordinated attack within our own borders.
It is a wicked world we live in, we are the number one Superpower, and we'll probably be coupled with Great Britain, with China and Russia nipping at our heels.
Already we are threatening Iran over their nuclear buildup. We got out of the last Depression with a World War. I honestly think that is what is coming, and it will begin in the Middle East.