Mr. Trump fails to take into account the major hurdle the wall faces: eminent domain. To build the wall, the U.S. would need to own all 1,954 miles of the border. Most of this land is now private property—especially in Texas, where the U.S. government owns only 100 miles of the 1,254-mile border. To acquire the rest of the land it would need, Washington would need to employ eminent domain, the authority under the Fifth Amendment to seize private property for public use upon payment of “just compensation.” Recent history shows that’s easier said than done. In 2006 Congress passed the Secure Fence Act with strong bipartisan backing, including the support of New York Democrat Chuck Schumer, now Senate minority leader. The law authorized construction of a border fence along 700 miles of the U.S.-Mexico border, including 100 miles in Texas. Lawmakers expected swift completion of the project. Instead, a decade later, there are unfenced gaps—because the fence had to have holes to accommodate local ranchers whose cattle graze on the southern side, but also due to property owners’ fighting land seizures in federal court.

At the end of 2016, more than 120 separate cases pertaining to eminent-domain seizures for the fence were still active in the U.S. court system. In 2009 the Department of Homeland Security inspector general issued a report that noted, “Acquiring real property from non-federal owners is a costly, time-consuming process requiring negotiations and sometimes condemnation.”