President Donald J. Trump may reject the credentials of the far-left British government’s incoming ambassador to the U.S. Labour Party grandee, Lord Peter Mandelson, who previously branded the American President a “bully,” a “danger to the world,” and “little short of a white nationalist and a racist.” Prime Minister Sir Keir Starmer’s choice of the scandal-ridden “Prince of Darkness” for the role generally assigned to career diplomats is inexplicable to many. Mandelson had to resign in disgrace from Tony Blair’s Cabinet on two occasions, only to be given a de facto promotion to the European Commission, where he was also...