An anonymous group of conservative billionaires is ready to place their bets on a man dubbed “Mad Dog,” hoping to draft him into the presidential race to confront Donald Trump. Think of it as a Plan B should Trump be nominated by the Republican Party in Cleveland: swing behind retired U.S. Marine Corps Gen. James Mattis and press him into service yet again as a third-party candidate. Mattis is the former commander of Central Command, which includes the strife-afflicted conflict zones of the Middle East, North Africa, and Central Asia, and has developed a reputation among troops as a general...