DEIR EZ-ZOR, Syria — Standing on a windswept mound, Brig. Gen. Jonathan Braga pointed to the spot just a mile away from where Russian mercenaries and fighters loyal to the Syrian government fired at his soldiers. "Those artillery rounds could have landed and killed Americans, and that's why we continue to prepare our defenses," said Braga, the director of operations for the U.S.-led anti-Islamic State coalition. U.S. troops who came under fire from Russian mercenaries prepare for more attacks Only steps away sits a sprawling oil and gas refinery that American forces captured from ISIS last year and that is...