I came across this on Eric Alterman’s Altercation, in one of the letter from Ken G of Cherry Hill, NJ that he published, detailing the basics of combat related psychological stress:

Studies during World War II determined that the average rifleman in combat maintained peak efficiency up to about 90 days of cumulative combat. Thereafter, efficiency decreased and ultimately, if not otherwise wounded, they became “psychiatric casualties” after 200 to 240 cumulative days in combat. Once they became “psychiatric casualties,” soldiers could be treated and made useful again in rear echelon duties but they were rarely able to become effective combat rifle men again. It was further determined that “Rest and Recuperation” (R & R) could “stop the clock” on the cumulative days of combat, but it could not “turn back the clock.” In other words, whether a rifleman served in combat for 240 consecutive days or for 240 single days with R & R in between, they were still likely to become psychiatric casualties. Interestingly, studies during the Vietnam War initially did not seem to support the figures calculated in World War II. However, further study determined that actually the figures from World War II were correct, but, since in Vietnam a soldier was invariably a target to insurgent snipers and mortar barrages and bombs whether on a search and destroy mission or at base camp or on R & R in Saigon, the “combat clock” continued to tick even during times that were seemingly non-combat. In others words, it is not the least bit shocking that most of the men who served as riflemen for a one-year tour in Vietnam suffered from some level of PTSD upon completion of their tour.

Note that the surge required 15 months tours, so people in the military and the administration knew that they were breaking the bulk of the soldiers who are so deployed.