The Obama administration's plan to raise a 15,000-strong rebel army in Syria has run into steep political and military obstacles, raising doubts about a key element of the White House strategy for defeating Islamic State militants in the midst of a civil war.. Pentagon concerns have grown so sharp that Defense Secretary Chuck Hagel sent a two-page memo to the White House last week warning that the overall plan could collapse because U.S. intentions toward Syrian President Bashar Assad are unclear, according to a senior defense official who read the memo but was not authorized to speak publicly. At a...