The U.S. commandos careened between grief and anger as Alex Conrad’s flag-draped coffin was hoisted onto the cargo plane. Al-Shabaab militants allied with al Qaeda had killed Staff Sgt. Conrad at a rudimentary outpost that American Green Berets built with help of the Somali army. Now, just as Sgt. Conrad’s comrades were sending him on his final trip home, they were grappling with more bad news. The Somali army had abandoned the outpost hours after the sergeant died. The Green Beret commander frantically called a senior Somali official…