The stats that Dr. Merali gave you are consistent for the military: about a third of the people who go into treatment for post-traumatic stress disorder are successfully treated, a third don't respond and will never respond, and a third stay in treatment.
Of the ones who are successfully treated, many do return to combat, if that is what is decided for them. The incidence of relapse is no greater than with any other mental health issue, if they have been successfully treated.