WASHINGTON, Aug. 6 - In a classified briefing to senior Pentagon officials last month, the top American commander in the Middle East outlined a plan that would gradually reduce American forces in Iraq by perhaps 20,000 to 30,000 troops by next spring if conditions on the ground permitted, three senior military officers and Defense Department officials said this week. The assessment by Gen. John P. Abizaid, the head of the military's Central Command, tracks with a statement made last week by the top American general in Iraq, Gen. George W. Casey Jr., that the Pentagon could make "some fairly substantial...