Former President Donald Trump’s huge win in the Iowa caucuses on Monday enshrines one of the most astonishing comebacks in American political history. Losing one-term presidents almost never mount subsequent successful primary campaigns, much less pull off landslides that demonstrate utter dominance of their party. Trump transformed the GOP in his populist, nationalist, nihilistic image in 2016. By claiming 50% of the vote in the biggest win in caucus history, putting him on course to his third consecutive nomination, he showed that eight years after his outsider presidential victory, the current GOP is entirely his party. “The big night is...