Why do so many returning soldiers have mental health problems?

Combat stress : In situations where one can be killed at any moment, combined with being awake for days on end and watching people die, psychological and physical effects as well as outright damage are lifelong in impact. See the first scenes of soldiers landing on Omaha Beach in the Movie " Saving Private Ryan" and imagine that playing over and over in your mind without letup. That will give you a rough idea.