But the expensive and useless wall has remained the centerpiece of Mr. Trump’s spiteful vision. From the outset, the project has been riddled with lies and illusions. Mr. Trump has argued that the wall would block illegal migrants, though as many as half of the undocumented immigrants in the United States entered legally and overstayed their visas. He claims that the southern border is a “pipeline for vast quantities of illegal drugs,” though according to the Drug Enforcement Administration most trafficking occurs through ports of entry — or, in the case of the powerful opioid fentanyl, through the mail.

It may well be that the $738 billion military budget for 2020 is too big, and that skimming a few billion off the top won’t affect American security. That misses the principle. The power of the purse belongs to Congress, and the lawmakers should decide how America’s wealth is spent. Despite its bitter divisions, Congress has repeatedly concluded that the wall Mr. Trump wants to build is a waste of money.