James Tot Joak hid in the swamp, keeping every possible part of his body submerged. He knew that a single cough, a single moan, a single cry from a baby or gasp of a child could bring death.

It was July 8 2016 and the Médecins Sans Frontières (MSF or Doctors Without Borders) primary health-care hospital in Thonyor had just come under fire as part of an offensive by government-aligned troops. Joak was a medical officer at the facility in the small village on the outskirts of Leer, the main town in southern Unity state.

He had just seen his colleague, a pharmacist, shot in the back and another colleague run over and crushed to death by a military vehicle. But Joak knew that the slightest misstep meant he — and dozens of others in the water with him, including patients — could be the next to face execution...