The principle reason why I don't think something like this is likely to happen, even in a big crisis situation, is that the American population is just getting too damn old. We're not Japan yet because the U.S. allows immigration to varying extents but nevertheless:
>The national median age rose to 38.2 years last year, according to newly released data from the U.S. Census Bureau. That suggests half of the U.S. population is more than 38-years-old and half is younger.
https://www.bloomberg.com/news/arti...s-are-now-over-the-age-of-38-census-data-show
Once you start getting into your thirties, if you don't actively work to stay in good shape you're going to age out of being able to do very much hard physical stuff pretty quickly – especially with the American lifestyle and diet. You can dress up in a fancy outfit but you're not going to be of much good if you can't run down the block.
Like that Dallas shooter from a few weeks ago? Who attacked that courthouse? He was in his early twenties and could barely tie up his boots. No surprise he got plugged by the cops and bled out in the parking lot. A lot of Americans also have a lot to lose – like how quickly did the alt-right evaporate after making all kinds of "race war durr we're gonna kill you" threats after people just started getting doxxed? Like if all it takes to abort the boogaloo is losing your job at Dick's Sporting Goods then there probably wasn't ever going to be a civil war in the first place.
>Between 1970 and 1999, 80 percent of civil conflicts occurred in countries where 60 percent of the population or more were under the age of thirty, according to the PAI report. Today there are sixty-seven counties with youth bulges, of which sixty of them are experiencing social unrest and violence. Demographers are quick to stress that youth bulges do not solely explain these civil conflicts—corruption, ethno-religious tensions, poverty, and poor political institutions also play contributing roles—but nor do they rule out as coincidence the predilection toward social unrest among states with large youth populations.
https://www.cfr.org/backgrounder/effects-youth-bulge-civil-conflicts