This article touches on how some of the first forms of PTSD seen was from the soldiers of the Civil War. It mentions how the devastation that many of the soldiers saw was unseen before and since they were subject to pre-modern medicine, their inflictions were difficult to treat. I think this is a great example of how the lack of information on mental health as well as medicine led to many patients being mistreated and misdiagnosed.
https://www.smithsonianmag.com/history/ptsd-civil-wars-hidden-legacy-180953652/
