WASHINGTON: The Pentagon said on Thursday it would not dispatch additional combat brigades to Afghanistan this year because of constraints imposed by the war in Iraq, leaving a shift of forces to the next president.nnUS commanders in Afghanistan have requested three more combat brigades, or about 10,000 troops, to deal with growing acts of insurgency in the eastern and southern parts of the country.n nGeoff Morrell, the Pentagon press secretary said improved security conditions in Iraq have raised the prospect for freeing up troops for Afghanistan next year, but Iraq remains the Bush administration's top priority.