Yeah, it's just a hypothesis; but based on my own experience with non-combat PTSD it makes sense to me that being unable to shut off the constant hypervigilance

Yeah, it's just a hypothesis; but based on my own experience with non-combat PTSD it makes sense to me that being unable to shut off the constant hypervigilance in the absence of a threat could easily be a common thread. The American Civil War was the first "modern" war as it made use of a lot of new technology that had been developed in the early industrial revolution, but barrel rifling is probably the most relevant to the experience of an individual soldier (as opposed to railroads, steam ships or the telegraph, each of which revolutionized the strategic dynamics of war).