He’s wheeled out of the shack where he’s been cowering, pale jeans now washed with red-brown blood, as a crowd bays for more. The shivering young Mozambican migrant, bleeding from a head wound, is protected from further assault by a squad of police officers, as he’s loaded into an ambulance in Alexandra township in Johannesburg. Parts of South Africa, especially in its biggest city, remain tense and occupied by security forces trying to prevent violence against foreigners. Protestors are demanding that migrants, who they accuse of “stealing jobs” and of being criminals, leave the country. America’s border crisis is in the south. South Africa’s is in its north. Migrants and refugees from a wide range of nations have for decades poured into the continent’s second-largest and most developed economy. The flood began in the early 1990s when apartheid began to fall. Some demonstrations have ignited attacks on people perceived to …