In terms of warfare, it is documented in some sources from the medieval period but quite rarely and mostly to do with protracted sieges.

There was a huge amount of study done on combat fatigue/ptsd/shellshock during and after both world wars. Medieval and ancient warfare were far less intense, as in you'd probably spend +90% of your time marching, waiting, building and only a tiny fraction actually fighting, you could go on an entire campaign and never actually be in mortal danger more than a few times. Where as in industrialised and modern warfare the level of intensity increased dramatically. Take ww1 as an example, instead of marching for months and having the occasional skirmish or settling in to besiege a town you are now put in a trench and constantly bombarded with machine guns, mortars, long range artillery and raiding at night. You can literally be killed at any time and are in constant mortal danger.

I think the research done around the wars came to the conclusion that after 2 months of this pretty much everyone breaks hence why regular r&r is essential to maintaining an effective fighting force.

There's also the conditioned response training modern armies use which makes a huge problem. In ww2 only half of us army troops actually fired their weapons. No normal person wants to kill, most people are capable of killing in a hand to hand scenario as it's literally him or me and this is a thing our brains can handle. But if you hand somebody a gun and tell them to shoot somebody over there that doesn't know you're there and isn't actively endangering your life, they won't want to do it, even if they do pull that trigger they will often consciously or subconsciously miss. That has been going on since the times of linear warfare.

Now in modern military training you take your soldier, a regular joe, and you run him over and over again through drills and combat simulations where targets pop up and they aim and shoot. You keep doing this until it becomes reactionary and part of their muscle memory. Now you stick joe here in a combat zone and of course all his training takes over, now when an enemy soldier pops around the corner at the end of the street Joe's mental conditioning comes into play and he'll raise that rifle aim and shoot on instinct pretty much skipping the thought process. Now Joe has just killed someone on reflex who may not have even been a direct threat to him when he pulled that trigger. That is something our brains cannot handle well