It was 4:30 a.m., and John Kahler and Zaher Sahloul, two American doctors, were shoehorned into an idling van in eastern Aleppo. They had come to this war-ravaged city to help treat its waves of wounded. Now, they were getting out.

The van was bound for Castello Road. It was the only route in or out of the besieged Syrian city, and it was paved with peril. On one side, rocket-propelled grenades. On the other, sniper fire. And overhead, the roar of armed jets and military helicopters.