JOHANNESBURG, SOUTH AFRICA — It was before 6 a.m., and Imtiaz Sooliman was already wide awake. That, in itself, was hardly unusual. Dr. Sooliman runs Africa’s largest humanitarian aid organization, a South African charity called Gift of the Givers Foundation, and describes himself as a man who gives “four thirds” of his time to his work. His days often begin before dawn and end near midnight, and his three phones almost never stop chirping and buzzing. “I never have to go looking for work,” he says. “The work comes looking for me.” 
But this was different.
On Dec. 6, 2014, Sooliman, normally a blur of movement in his green Gift of the Givers tracksuit, was the one waiting. Three thousand miles away from the bedroom of his Johannesburg, South Africa, home, a convoy of cars prepared to begin a long ride into the Yemeni desert. At the end of their drive …