To recap, the USA are increasing their number dramatically and we are reducing ours. The US controlled north of the country will be a much tougher place for terrorists and insurgents to operate. The south of the country will be become a much more benign environment for them.
Chances are the insurgents will just move south. I have said before that our troop withdrawals should be done for the right reasons, political advantage shouldn’t be one of them.
Again as pointed out by St Crispin, the end result will move from the original situation of having just enough troops in Iraq and not enough in Afghanistan to one where we don’t have enough troops in either theatre.