Mexico is now the wall. President Trump got his wish.

The heart-wrenching images documenting a recent confrontation in the state of Chiapas, near the border with Guatemala, are evidence of this. Dozens of Mexican National Guard troops equipped with helmets, batons and transparent shields coalesced on the highway connecting the Mexican cities of Ciudad Hidalgo and Tapachula to stop a caravan of migrants heading to the United States from Central America.

The guardsmen used pepper spray on the caravan, which as of mid-January included about 4,000 people, many of them women and children. In the end, hundreds were detained, sent back to Guatemala or deported to Honduras. A spokeswoman for the U.N. High Commissioner for Human Rights condemned the actions of the National Guard, saying that the use of force to stop and disperse immigrants should be avoided.

Mexico has effectively turned into an extension of Mr. Trump’s immigration police beyond American territory. And this is the case on multiple fronts: On the southern border with Guatemala, they prevent Central American migrants from coming into Mexico; on the northern one, they block those seeking entry to the United States from leaving. The decision of Mexico’s President Andrés Manuel López Obrador, also known as AMLO, to follow this approach is misguided. He should let migrants continue their journey north.

Under the Migrant Protection Protocols program, which the Trump administration introduced in January 2019, asylum seekers who attempt to enter the United States at the U.S.-Mexico border may be required to stay in Mexico while the authorities make a decision regarding their case. As of November 2019, over 56,000 people had been sent back to Mexico to wait for the outcome of their applications, according to Human Rights Watch.