WASHINGTON — President Donald Trump has become increasingly frustrated with his advisers tasked with crafting a new U.S. strategy in Afghanistan and recently suggested firing the war's top military commander during a tense meeting at the White House, according to senior administration officials. During the July 19 meeting, Trump repeatedly suggested that Defense Secretary James Mattis and Chairman of the Joint Chiefs of Staff Gen. Joseph Dunford replace Gen. John Nicholson, the commander of U.S. forces in Afghanistan, because he is not winning the war, the officials said. Trump has not met Nicholson, and the Pentagon has been considering extending...