Gates said the change is necessary to prevent five Army brigades from deploying to combat before they complete a desired 12-month rest period at home and to give predictability to soldiers and their families.The surge, of course, consists of five brigades. What Gates is telling us is that the Army didn't have those brigades, and the only way they got them was by lengthening the normal tour of duty in Iraq to 15 months.
....He said the longer deployments also will allow President Bush's "surge" strategy in Baghdad to last a year in case such high force levels are needed in Iraq.
It's not plausible that the Pentagon didn't know this when the surge was announced. They just decided not to announce it at the time. I'll leave it as an exercise for the reader to guess why.