Mostly because our modern civilizations were born there. "The region saw the development of some of the earliest human civilizations,
which flourished thanks to the water supplies and agricultural
resources available in the Fertile Crescent. Technological advances made
in the region include the development of writing, glass, the wheel, agriculture, and the use of irrigation." ~Wikipedia
About 11 to 20 out of every 100 veterans (or between 11 and 20%) who served in operations Iraqi Freedom and Enduring Freedom have PTSD in a given year.
About 12 out of every 100 Gulf War Veterans (or 12%) have PTSD in a given year.
About 15 out of every 100 Vietnam veterans (15%) were currently diagnosed with PTSD when the most recent study of them (the National Vietnam Veteran Readjustment Study) was conducted in the late 1980s. It’s believed that 30% of Vietnam veterans have had PTSD in their lifetime.
There isn't a exact percentage for all the veterans in the US but i hope this helps!
