Donald Trump “would be the most reckless president in American history,” according to a damning letter signed by 50 prominent Republican national security experts urging voters to stay away from the GOP nominee this fall. In the clearest sign yet that Trump has failed to win over droves of Republican voters worried about his mercurial temperament and erratic behavior, the letter’s signatories, which include dozens of former officials in the Richard Nixon, Ronald Reagan and both Bush White Houses, made clear, in no uncertain terms, that the mogul not only lacks the credentials to be the nation’s leader but would...