Sniper Fire Greets G.I.'s in Big City in North
American Special Forces troops enter Mosul, Iraq, after organized Iraqi resistance flees, finding city engulfed by anarchy; quickly retreat; incursion followed talks with tribal and Baath Party leaders intended to bring about peaceful surrender of city, homeland of Sunni Arab nationalism; armed Kurds in civilian clothes drive around city flying flag of Kurdish Democratic Party; thousands of Kurdish troops remain outside city; Kurds in city stage small-scale uprising; city's Arabs begin attacking Americans; Arabs and Kurds blame US for not moving soldiers into Mosul quickly enough to stop looting; photo (M)