What changed was the Migrant Protection Protocols. Launched as a pilot program in El Paso and San Diego in January, the Trump administration expanded and accelerated the MPP program, also known as “remain in Mexico,” as part of a deal struck with Mexico in June to crack down on illegal immigration from Central America. The program requires migrants seeking asylum in the United States to wait in Mexico while their cases are being adjudicated—a process that takes months. All told, the United States has sent nearly 43,000 non-Mexicans back across the Rio Grande to await their hearings.