BAGHDAD — The commander of a group of Syrian fighters trained by the United States has been kidnapped by Al Qaeda’s affiliate in Syria, his group said in a statement Thursday. The commander, Nadeem Hassan, and seven of his fighters were taken by the Nusra Front, a rival of the Islamic State in Syria, as they were returning from a meeting in Turkey. A contingent of 54 fighters from Mr. Hassan’s group, known as Division 30, are the only ones who have graduated from a Pentagon program to train Syrian rebels to fight the Islamic State in Syria. One the...