Theres a lot more combat in Iraq than in Afghanistan. Some 75 percent of the troops coming back from Iraq have been exposed to enemy fire (bullets, RPGs, roadside bombs or mortar/rocket attacks on their base.) Only 31 percent of the Afghanistan vets have been so exposed. But the Afghanistan combat tends to be more intense. About twenty percent of the Iraq troops exposed to combat, report having some psychological problems as a result. But some 35 percent of the Afghan combat veterans have those problems. 

Psychological after-effects of combat have been noted as far back as the American Civil War (1861-65). But it was only after World War II, when there were so many cases, and so many serious ones (including a few vets who went berserk with a firearm), that the problem got some serious attention.  The 1950s saw the introduction of tranquilizing type drugs (which have evolved into things like Valium), and this did a lot to help deal with the anxiety many of the combat veterans were suffering from. Since these vets were the first to be studied, in detail, and on a large scale, a lot more is known about the combat fatigue syndrome. While its called Post Traumatic Stress Syndrome these days, the military cases form a distinct version of this affliction. For most troops, time is the ultimate cure. For many World War II vets, it took twenty years or more to calm down. With immediate treatment, the problem is less severe, and goes away more quickly. But theres a problem with getting combat veterans to admit they have a problem. Many see such an admission as harmful to their career prospects. Overcoming this kind of fear is often more difficult than dealing with the combat fatigue itself.