To find the answer, this writer came to Ciudad Acuna, the city through which the migrants had passed to form the encampment and asked several dozen of them what happened. The surprising answer, which the migrants provided independently in different places and at different times, was universal: on Sunday, September 12, the Mexican government effectively sent a mass of migrants it had bottled up for months in its southern states up to the American border. This move, which appears to have been done under the cover of Mexico's independence week of celebration known as El Grito, essentially foisted a humanitarian...