I would speculate that it has always been with us in centuries and millennia past. Here are some writing arguings that we see descriptions of what we now call PTSD in Ancient Greece and Mesopotamia through to the US Civil War and today:

https://medium.com/lessons-from-history/the-hidden-evidence-of-ptsd-in-the-ancient-world-eb83752b7d4e

https://militaryhistorynow.com/2012/09/17/walking-wounded-ptsd-from-ancient-greece-to-afghanistan/

Evolutionarily speaking, it makes sense. The world was more violent in the past and many men would have been exposed to horrific violence and death of close comrades. Our evolved psychology was the same then as now, so I would guess our response to war was the same too.