Clausewitz (On War, Chapter XII) has this to say about pursuit of a defeated enemy:
But under any conceivable circumstances the fact holds good, that without a pursuit no victory can have a great effect, and that, however short the career of a victory may be, it must always lead beyond the first steps in pursuit...Clausewitz's reasoning, though drawing on battlefield examples from evenly matched armies of the Napoleonic Wars, is as valid today. Considering the ongoing struggle in Iraq, where recent events have the tang of victory, shows what a colossal strategic mistake is being made in drawing down our troops in response.
Usually victory at the moment [of its realization], even if it is certain, is still as yet small and weak in its proportions, and would not rank as an event of any great positive advantage if not completed by a pursuit on the first day. Then it is... that the trophies which give substance to the victory begin to be gathered up.
This is the time to redouble our effort in Iraq, to increase the pace of operations still further against a weakened enemy, and to reap the advantages for whose growth our soldiers have toiled and bled. By drawing down our forces, we are pursuing not victory, but an ugly sort of draw -- a continuation of the current violence, perhaps at a slightly lower level.