What you need to do is create a country wide hot box. If they are forced to inhale they will definately cheer up. Or make em brownies and fail to mention what kind of brownies they are.
I doubt the US soldiers would take martial law that far. I'm sure they would kill some of their own people, but not for wearing shorts. Maybe for causing other crimes like rape or something in the chaos. And I seriously doubt they would kill them for possesion. They would prolly just ask. These are just regular soldiers. If there was a draft, anyone of you can be one of them. And by the way, I think there would be a court marshal if they went and killed someone because of their shorts.