7:32 p.m. ET Jan. 26, 2017   Cracking down on visa overstays and on employers would be more cost-effective.

 Since 2005, the federal government has added hundreds of miles of walls and fencing along the U.S.-Mexico border. It has doubled the size of the Border Patrol by hiring more than 10,000 agents. And it has ramped up spending so rapidly that it is plagued with duplicative programs.

So when President Trump says he is moving ahead with a massive border wall, it has all the hallmarks of a multibillion dollar boondoggle. And his insistence that Mexico be forced to pay for his costly campaign pledge threatens to rupture relations with an important ally and trading partner.

Physical barriers certainly have a significant place in border security. But any major expansion of the existing barriers should be done in the context of cost-benefit analysis. By any reasonable accounting, the surge of spending on border enforcement has already reached a point of diminishing return.

The federal government now spends more policing immigration than it does on all other law enforcement activities — combined. More, that is, than on drug trafficking, gangs, counterfeiting, identity theft, financial fraud, would-be assassins, routine interstate crime, illegal arms sales, computer hacking, corporate malfeasance, government corruption and the domestic part of the war on terror.

Most of California, Arizona and New Mexico already have some kind of barrier. Texas is another matter, thanks to the difficulties of building along the snaking, flood-prone Rio Grande River, and the fact that much of the border land is in private hands.

Since 2007, the estimated number of undocumented immigrants has dropped from 12.2 million to slightly more than 11 million, thanks to some combination of increased enforcement, declining birth rates and rising economies, particularly Mexico's.

This isn’t to say illegal immigration has stopped outright. But it is being offset by people returning to their home countries. What’s more, an estimated 35% to 50% of the inflow is people who come in legally and overstay their visas, people who are not impacted by walls or other border control efforts.

Taking all this into account, Trump's wall would be a colossal waste of money. His idea of forcing Mexico to pay for it has already led to cancellation of next week's scheduled meeting between Trump and Mexican President Enrique Peña Nieto. Slapping a 20% tax on imports from Mexico, which a Trump spokesman floated Thursday as a way to recoup the construction costs, would set off a mutually destructive trade war and effectively make U.S. consumers pick up the tab.

Cracking down on visa overstays and on employers who hire illegal workers would do far more to improve immigration enforcement than spending an additional $12 billion or more on steel and concrete.