When the 82nd Airborne division left the Sunni Arab region, and was replaced by the marines, lots of data on who is who in places like Fallujah was turned over. Some army intelligence experts stayed behind to work on some of the continuing  information gathering operations. The armed gangs in places like Fallujah are a complex brew, since nearly everyone has a gun and a large percentage (perhaps as many as twenty percent of adult men in places like Fallujah) are willing to use them. Moreover, the gangs (some of which are clan or family organizations) are always jockeying for position. There are also illegal rackets (extortion, smuggling, kidnapping) to fight over as well. The marines are using aggressive patrols  to gain undisputed and 24 hour control of the streets. This involves a lot of night operations, where the marines have an edge with their night vision equipment. The Sunni Arabs do not have to come to terms with the marines, they have to come to terms with the rest of Iraq, and themselves.