Will the courts let Donald Trump build his wall?

A FEW HOURS after Donald Trump tweeted about “the attempted Invasion of Illegals, through large Caravans, into our Country”, Carmen sat in a church office in suburban Maryland, quietly sobbing. Five years ago she and her son, who was then three, fled from El Salvador and her violent husband. “It’s difficult to leave your country,” she says. “You have your family and your friends. Your whole life is rooted there. But when it comes to your child’s safety, I don’t think there is anything a mother wouldn’t do.” They crossed Guatemala, Mexico and the Rio Grande before presenting themselves to immigration cops in New Mexico. After being detained, she applied for asylum, and was released to await a hearing. She is still waiting.

If America has a border crisis, it comes not from any sort of invasion—in the year to September 2018, the authorities caught 396,579 people trying to cross the southern border, fewer than half as many as in 2007—but from people like Carmen and her son: families fleeing troubled states in Central America to seek asylum. Mr Trump’s steel-bollard fencing, even if it survives a legal and political assault, will do nothing to fix that problem.

Mr Trump wants to add 234 miles of fencing to the roughly 700 that already exist along America’s border with Mexico. He has identified four sources to pay...