Donald Trump has described at length the dangerous and disturbing actions he says he will take if he wins the presidency. His rallies offer a steady stream of such promises and threats — things like prosecuting political opponents and using the military against U.S. citizens. These statements are so outrageous and outlandish, so openly in conflict with the norms and values of American democracy that many find them hard to regard as anything but empty bluster. We have two words for American voters: Believe him. The record shows that Mr. Trump often pursues his stated goals, regardless of how plainly...