In early January, I drove along the Pan-American Highway in the scenic Mexican state of Oaxaca. On the opposite side of the road, the Mexican National Guard had erected a temporary roadblock. A line of cars heading north had halted. Uniformed officers walked down the line, questioning drivers. They were searching for migrants bound for the United States. A few hours later, I returned by the same route. I braced myself for the obstruction and delay. There was none. The roadblock had vanished. In the effort to contain unauthorized migration to the U.S., Mexico is an on-again, off-again partner. Sometimes it helps more; sometimes less. In 2022, Mexico detained almost 320,000 migrants and expelled 106,000, according to a condemnatory report by Amnesty International. Detainees were held under conditions much harsher than would be allowed in the U.S. A migrant from El Salvador described the facility where he was held in Ciudad Juárez, the Mexican border town adjoining El Paso, Texas. As NPR reported in 2023: There was no water and scant food. There was no …