I would say that logotherapy (Frankl's method) is well-suited to helping people who either are or have been in traumatic situations such as war. Frankl's ideas were formulated while he himself was spending time as a prisoner in Nazi death camps, which is a pretty traumatic experience itself. While in the camps, Frankl said, it was usually easy enough to see who was actually surviving and who's number would soon be up. The one's who were about to give up the ghost, so to speak, were those who had lost all meaning. They no longer felt that they had something to live for.
According to Frankl, the most powerful and empowering thing a person can have is something to live for. This overarching life purpose can strengthen someone to endure and triumph over any kind of difficulty or trauma, whether present or in the past (as is the case with PTSD). Thank you for the good question.