The sun beat down on Rebeca Escala’s hands as she gripped a sign at a roundabout in this border city: “Tijuana, home of immigrants.” Around her, about a dozen people waved multicolor flags and handwritten placards denouncing xenophobia. Suddenly, the small group dispersed. About 100 anti-immigrant protesters who had gathered at another traffic circle down the block, many of them wearing black, were making their way toward them, yelling, “Out, Hondurans! We don’t want you here!” At times sprinting and eventually growing to nearly 300 total, the self-described nationalist demonstrators reached the perimeter of the shelter, where 2,526 recently arrived...