When you take a serious look at it, it's not just the Stress that is the major factor, it's the Anxiety that the majority of military personnel have trouble with. I have always explained PTSD as an umbrella term for a myriad of symptoms. For instance nearly everyone with PTSD gets a diagnosis of PTSD with an Anxiety disorder and some degree of depression, and when you look at the ailments that follow, you get bruixism, IBS, hypervigilence, etc, etc, and not everyone suffers from the exact same symptoms or intensity. Therefor it's a disorder.
Lets just call it combat stress and be done with it. It's not an injury. It's the mind not being able to cope with what it is seeing, that is why if you catch a young veteran early enough and they have only had one deployment, their is a great chance they can return to an almost normal life. But when you add trauma after trauma and deployment after deployment, there is little hope. They major key is debrief and counselling. Its not plausible, but if they were to debrief a veteran after each episode where there was trauma, the amount of veterans coming home with PTSD would be greatly reduced. Have a look at our day to day emergency services, they still get cases, but the numbers are minimal, why, because they debrief after every major incident.
Compulsory debrief training carried out over a few days at the end of deployment followed up by another period three months after deployment would catch quite a few too. Ask any veteran with PTSD and they would agree.