Soon after his country began bombing Yemen in 2015‚ the de facto ruler of the United Arab Emirates confidentially reached out to an old friend: retired Marine Gen. Jim Mattis. Sheikh Mohamed bin Zayed al-Nahyan‚ who also served as the deputy supreme commander of the Emirati military‚ needed help. The UAE was part of a coalition of Arab countries that had intervened in Yemen’s civil war to fight Iran-backed Houthi rebels. But the coalition’s bombing campaign was killing large numbers of...