The main Western-backed Arab rebel group in Syria appears on the verge of collapse because of low morale, desertions, and distrust of its leaders by the rank and file, threatening U.S. efforts to put together a ground force capable of defeating the Islamic State and negotiating an end to the Syrian civil war. "After five years of this war, the people are just tired ... and so are our fighters," said Jaseen Salabeh, a volunteer in the Free Syrian Army, which was formed in September 2011 by defectors from the army of Syrian President Bashar Assad. The Free Syrian Army,...