As at Nasiriyah, much of the fighting occurred in places through which elements of the U.S. military had already passed in recent days. That raised the question of whether Iraqi military leaders had initially lost communications links to the field and had now regained them. Another possibility is that Iraqi forces, knowing they would be slaughtered in a head-on fight with a far-more-powerful foe, hid in the cities, waiting to ambush more-vulnerable U.S. support troops following behind. Besides endangering troops, the fighting in the south has so far deprived the U.S. of an important psychological weapon. Going into the war, U.S. planners hoped that American troops would be greeted by joyous civilians in the southern city of Basra and elsewhere that would be broadcast around the globe. U.S. officials expected to use the resulting pictures to help persuade Iraqi troops and civilians elsewhere to abandon Mr. Saddam Hussein and his regime. The division near Karbala already was responding to the U.S. assault with artillery fire aimed at advancing U.S. troops from the Army's Third Infantry Division. U.S. warplanes hammered guard divisions Monday with 2,000-pound satellite-guided bombs. The planes also hit two other Republican Guard units on the immediate southern outskirts of Baghdad.