The rebel fighter in charge refused to let the strangers pass their position. Gun battles were rocking Aleppo’s Bab al-Nasr neighborhood, and the road ahead was packed with government snipers who shot at anything that moved.

The rebel also doubted the strangers’ motives. What were these two men doing at his frontline position, unarmed, with nothing more than white armbands and a plastic refrigeration box slung on their shoulders?

Ahmad Jamal, 34, explained that his team needed to vaccinate the local militia commander’s children. Polio was a highly contagious virus, he noted, and even one overlooked child could infect hundreds of others.