U.S. Marines rolled into the heart of the Sunni stronghold in May, seeming to quiet the restive city of Fallujah after terrorists attacked and hanged from a bridge support three U.S. contract workers. Six months later, however, Fallujah was once again a safe haven for terrorists like Abu Musab al-Zarqawi, who oversaw a series of grisly beheadings there. Coalition forces learned a lesson: Don't put in charge former Baathists, like the local Iraqi commander dubbed to subdue Fallujah. So in November, the Marines were back-15,000 strong when combined with their Iraqi counterparts. The year's biggest offensives sent terrorists running from the city and yielded a treasure trove of records-and a firsthand look at Zarqawi's slaughterhouses-that will aid in hunting down insurgents for months to come.