The United States has provided military support to Saudi Arabia, which leads a coalition of Arab states that entered the war in March 2015 to beat back what Saudi Arabia said is an Iranian proxy force. While analysts said at the outset of the conflict that the Saudi claims were exaggerated, most agree the war has driven Shiite Iran and the Houthis toward greater cooperation.

"Most people agree at this point that the Saudis are facing a legitimate security threat and that Iran is part of the problem," said April Alley, a Yemen researcher at the International Crisis Group.