The migrant caravan is like something out of a future, apocalyptic dystopia—or, to go back in time, perhaps an image from Exodus—where thousands of men, women, and children trudge a thousand miles under the hot tropical sun across hot tarmac and dirt roads, hoping to land on America’s doorstep, in search of opportunities not to be found in the failed, crumpled, venal, socialist states from which they hail.What is President Trump to do? Welcome them all in, as one interpretation of Pope Francis’s recent exhortations seems to imply? Turn them back? If so, how? Persuasion? Water cannons? Trump has...