I thought that WWI is when "shell shock", as it was then caused, started to become a big topic? You're probably right that _most_ soldiers from WWI (at least the ones who came back alive, and survived the Spanish flu) didn't develop PTSD, but in ancient times it seems like it was unheard of as opposed to unusual.
I was always told in my family that my great-grandfather was "never the same again" after he came back from WWI, and I think PTSD in some form is the most likely explanation.
Helplessness as an explanation checks out there as well. Hunkering down in a trench or a foxhole as shells rain down endlessly, blowing up everything around you. Sounds like one of the most powerful ways to inflict abject helplessness.
Compare that to the ancient battlefield where you had a spear in hand and your friends at your side. You’d have a lot more sense of agency and if things looked really bad you could run away or surrender. Of course if your opponent was rounding up and executing all of the prisoners then you would feel pretty helpless waiting for your turn, but then you wouldn’t live to tell the tale of your trauma.
WWI was also arguably when soldiers became helpless.
Being deployed meant huddling in a trench waiting for an artillery shell to strike you out of the blue.
For about the previous two centuries, the typical soldier's experience was endless marching and then standing in formation with hundreds of others, shooting very imprecise guns at similar enemy formations and hoping that none of the hundreds of enemy bullets hit you despite a complete lack of cover.
I was always told in my family that my great-grandfather was "never the same again" after he came back from WWI, and I think PTSD in some form is the most likely explanation.