Reduced violence only means that the bad guys are staying low or moved elsewhere (and can come back). It can also mean, which is the part of the strategy, that agreements have been brokered with the Sunnis, which you will need as a counterweight against the Shiites. It does not mean that the problem is resolved (potential for wide scale ethnic violence ala Kosovo/Serbia.) Historically, insurgency, especially Islamic ones, can stay there forever. The US first encounter Moslem insurgency via the Moros in the Philippines in the early 1900s (why the 1911 handgun was made), and their descendants of that insurgency, under names like the MNLF, are still there a straight century after, despite training and arming the Philippine armed forces. Personally however, is the insurgency does not appear to be in danger of toppling the government, then let the local government handle it. Concern yourself elsewhere. We once thought that the Taliban was finished, now they’re coming back. Exactly like my first sentence. But you can always bring back forces if problems escalate.

The new Iraq is getting too attached to the US via an umbilical cord. That cord has to be cut and the only way to know if they can make it on their own, is let them go.