The answer is clear from recent history. When the Marines leave, order and rule of law will quickly disappear. It happened in Somalia and when the Marines left Haditha for Falluja. When the Marines leave Iraq, every two-bit terrorist will come out of the woodwork.
At least three of the accused Marines were sent to fight in Fallujah and then back to Haditha. If the terrorists hadn't taken over, the Marines might never have been ambushed.