President Trump reiterated his threat Saturday to “close the border” or “large sections of it” next week. To put it mildly, “closing the border” is a terrible idea. The president cannot close the border to illegal immigration. What he can do—and is already doing—is slow the flow of legal crossings.

• Mexico is America’s 2^nd largest export market and 3^rd largest total trading partner.
• U.S. border crossings with Mexico handle a half a trillion dollars in trade each year. Nearly half enters through Texas where two thirds of illegal border crossings have occurred this year.
• Each year, U.S.-Mexico border crossings permit the entry of more than 190 million people and 6.3 million commercial trucks.
• Nearly 5 million American jobs depend on U.S.-Mexico trade.
• Mexico exports to the United States $26 billion in intermediate goods (components of final products), meaning that closing the border would be like building a wall in the middle of a factory floor.

Even a temporary slowdown would have significant effects:

• The temporary closure of the San Ysidro port in California last year—in order to tear gas migrants trying to cross—cost businesses $5.3 million.
• During that shutdown, traffic diverted to the Otay Mesa port resulting in 5 hour waits there.
• Normal lengthy waits to cross into the U.S. resulted in lost output of $6 billion in 2009 in California alone.

Other presidents have intentionally slowed traffic at ports of entry, but none since the implementation of NAFTA in the 1990s. Even then, the economic costs were significant. As the Boston Globe described President Nixon’s insane drug control strategy in 1969:

Cars and trucks were all checked for illegal contraband, and, as the day wore on, drivers who’d waited for hours without air conditioning or water were strip-searched. Over the next two weeks, as tens of millions of dollars evaporated from the economies of the border towns, Nixon came under enormous political pressure to back off. La Prensa — San Antonio’s bilingual newspaper — headlined one of its front pages “Humiliating Mexicans,” and Diaz accused Nixon, in terms that would be particularly ironic nearly 50 years later, of creating a “wall of suspicion” between the two countries. Almost no marijuana was seized.

Allan Golombek’s description in Real Clear Politics of the results of the GM Canada strike may also be revealing:

The three-week long GM Canada strike in 1996 serves as a good example of what actually happens when imports stop flowing into the United States. The strike by the Canadian auto plant didn’t just idle four assembly plants and numerous other facilities in Canada. Because GM operations were tightly integrated even then, the interruption of the flow of parts and vehicles had a swift domino effect, almost immediately prompting the company to close plants and lay off close to 2,000 workers in Michigan and upstate New York. Many other GM plants in the United States were subsequently slowed and stopped. Auto analysts estimated the automaker was a day or two away from having to shut down most of its North American operations when the strike ended after three weeks.

President Trump’s strategy is to scare the Mexican government into stopping Central Americans from coming North. But the Mexican government is already desperately trying to do just that based mainly on its own internal political considerations. Last week, even before these threats, it stopped issuing humanitarian visas to Central Americans and deployed its military to create an inland checkpoint line across the Isthmus of Tehuantepec—the narrowest point of Mexico.

Mexico’s prior attempts to control the flow have only achieved temporary results as it only forces smuggling networks to adjust their strategies. Ultimately it will take changes to U.S. visa policy to provide an alternative to traveling through Mexico to permanently reduce the flow.