Sviatlana Tsikhanouskaya, an opposition leader in Belarus, hadn’t heard from her husband Siarhei in more than two years. Not since Belarusian authorities placed him in “incommunicado” detention, in full isolation. And then he called her. “My dear wife,” he said. “I’m free.” He had been released from prison after negotiations between authoritarian leader Alexander Lukashenko and Trump administration officials, and was at the border between Belarus and Lithuania. He and 13 other political prisoners were heading to the American embassy in Vilnius. Tsikhanouskaya and Siarhei have since reintroduced him to their children, traveled to a solidarity rally in Poland and...