WASHINGTON — An American kidnapped two years ago in Yemen while helping coordinate aid for Unicef and the Red Cross also had a second, secret role: He was shipping materials for elite military commandos under a clandestine contract his employer had with the Pentagon. The arrangement with Special Operations forces has never been made public. {snip} Mr. Darden’s work offers a rare look into the shadowy world of military contractors that operate in lawless war zones like Yemen, Somalia and Libya. But arrangements like the one Transoceanic had with Special Operations forces can cast suspicion over aid workers, potentially putting...