BAGHDAD, Iraq - U.S. soldiers, automatic rifles buckled to their body armor, filed into a community center in a dangerous Shiite neighborhood of north Baghdad Saturday and for a few hours became social workers, cops on the beat and referees between feuding tribesmen. Tea was passed around, notes were taken, local sheiks spoke in wise tones, heads nodded vigorously in agreement and mundane problems such as garbage collection and distributing electricity generators were tackled. Maj. Michael Fazio, 36, from Warwick, N.Y., pulled out a cheat sheet — photocopied snapshots labeled with long tribal names that he tried to match with...