Right. Loads of Americans don't realize that a lot of people in their communities are members of some kind of emergency reserve component. And a lot of this sort of "hidden infrastructure" would mobilize relatively very quickly (on the scale of hours). In addition to slogging through tours overseas in shithole countries, I spent twelve years of my life preparing for large-scale natural or man-made disasters. People just don't realize how much of the community would mobilize in the event of an emergency.
I'm not sure how you mean "mobilize". If things weren't too bad, that would be a good thing. But if things got really bad, at least some of them would go feral. Trained and armed. Or just be looking for work and food.
This scenario is a fun thought experiment in science fiction stories, but this is so astronomically unlikely, you can plan around it. It is fairly stupid to make plans for this kind of scenario, because you will probably die before it gets that bad. You live in a city? You will be dead long before "things got really bad". The worst case scenario worth planning for is a totalitarian government. Martial law that gets extended indefinitely, and a general that becomes "President". There would be famine generally, and loads of people will starve to death. But independent roving warlord raiding bandit tribes ("feral" authorities) like in Mad Max? Not happening.
