Not long ago it would have been unthinkable: a Sunni sheik allying himself publicly with American forces in a xenophobic city at the epicenter of Iraq's Sunni insurgency. Today, there is no mistaking whose side Sheik al-Rishawi is on. Outside his walled home, a U.S. tank is on permanent guard beside a clutch of towering date palms and a protective dirt berm. The 36-year-old sheik is leading a growing movement of Sunni tribesmen who have turned against al-Qaida-linked insurgents. The dramatic shift in alliances may have done more in a few months to ease daily street battles and undercut the...