CAMP LIBERTY, Iraq — It was 3 o’clock on a January morning when a convoy of Stryker vehicles rolled out of this protected American base near the main Baghdad airport. The discovery of 27 bodies on a part of Haifa Street, a meandering road that also goes through the Green Zone or, as it is now called, International Zone, meant it was time to help the Iraqis take back what has been called one of Baghdad’s meanest and toughest areas, a place, according to news reports, even henchmen of former Iraqi dictator Saddam Hussein avoided. Sgt. Holly Jensen of the...