Sorry to link to a reddit thread on this. I was checking in on enemy coms over there when I saw this juicy planefag thread and thought I should bring it to y'alls attention.
https://old.reddit.com/r/UFOs/comments/14z4k8s/us_army_has_just_launched_helicopters_from_every/
Going to the US Code cited in the order, it seems like he can activate some reserve units unilaterally.