NAJAF, Iraq — Deafening noise, confusion and fear erupted as the roadside bomb slammed into the U.S. Army Humvee, knocking over Spc. Stephen Monti, who was manning a gun in the turret.

The military’s High Mobility Multipurpose Wheeled Vehicle (HMMWV), or Humvee to most of us, is a rugged four-wheel drive vehicle designed and manufactured by AM General. In basic trim, the truck ...