WASHINGTON -- Unless Iraq's military capitulates quickly, the most difficult phase of a war to overthrow President Saddam Hussein will likely be the battle for Baghdad. American military triumphs since 1990 have taken place in the open desert of Arabia, the airspace over the Balkans and the barren plains of Afghanistan. Fighting in cities is a different matter; exercises and simulations show it remains an enormous challenge even for today's U.S. armed forces.