The sign was ominous, the humor dark. Iraqis who live in the neighborhood had suddenly vanished, often an indication that an attack is imminent. "No way is anything going to happen," joked Staff Sgt. Angel Perez, a Humvee commander, as he watched an Iraqi police convoy drive near the outpost of the platoon from the 3rd Armored Cavalry Regiment. "That convoy is definitely not going to get hit," he said. Seconds later, a roadside bomb detonated just ahead of the convoy. Nobody was hurt, but the explosion shook the walls of the barracks and sent the men running for weapons and roaring out of the gate in Humvees. Although security in Iraq has improved, it remains fragile, Lt. Gen. Lloyd Austin, the No. 2 U.S. commander in Iraq, told reporters in the capital Monday. And nowhere is the fragility more apparent than here in Mosul, 225 miles northwest of Baghdad. Sunni Arab insurgents, routed or weakened in Baghdad and other urban centers since last year, are making a stand in this former bastion of support for Saddam Hussein. ...