Many observers were skeptical that such a deal could be struck, even if only informally. And it may yet unravel, especially if Trump further antagonizes Mexico’s nationalist government, which is keenly aware that its ability to control migrant flows gives it a great deal of leverage. One way or another, the U.S. needs to reform its outmoded and overwhelmed asylum system. What Mexico has done is give the Trump administration much-needed breathing room, and a path to a more cooperative relationship.

But rather than cement this fragile compromise with Mexico by, say, showering López Obrador with praise and calling for a new immigration agenda that would bring inflows under control by spreading prosperity throughout the region, Trump has chosen a different course of action. Once again, he will rage against congressional Democrats for refusing to fund a border wall that they feel quite confident in opposing, the heroic efforts of Vice President Mike Pence and the ubiquitous Mick Mulvaney notwithstanding. Then, after an interval of uncertain duration, he will cave as the political toll mounts, just as his detractors on the right are now predicting. Restrictionists have every reason to question their faith in their decidedly flawed champion.