On 8 March, Immigration and Customs Enforcement (Ice) agents arrested Mahmoud Khalil, a graduate student at Columbia University, as he was returning from dinner with his wife in New York. The agents said the state department had revoked his student visa and green card, though he had never been accused of, let alone convicted for, a crime. He was held in detention in New Jersey, then transferred to Louisiana. He has still not been accused a crime. Nevertheless, Donald Trump’s state department, headed by Marco Rubio, seeks to deport him under a provision of federal law that gives him the...