It is either national suicide, the desire for self-annihilation, or just another screw up, or perhaps a Deep State conspiracy, but the US is supplying weapons to terrorists, that in turn are used against the US:
https://edition.cnn.com/interactive/2019/02/middleeast/yemen-lost-us-arms/
“Saudi Arabia and its coalition partners have transferred American-made weapons to al Qaeda-linked fighters, hardline Salafi militias, and other factions waging war in Yemen, in violation of their agreements with the United States, a CNN investigation has found. The weapons have also made their way into the hands of Iranian-backed rebels battling the coalition for control of the country, exposing some of America's sensitive military technology to Tehran and potentially endangering the lives of US troops in other conflict zones. Saudi Arabia and the United Arab Emirates, its main partner in the war, have used the US-manufactured weapons as a form of currency to buy the loyalties of militias or tribes, bolster chosen armed actors, and influence the complex political landscape, according to local commanders on the ground and analysts who spoke to CNN. By handing off this military equipment to third parties, the Saudi-led coalition is breaking the terms of its arms sales with the US, according to the Department of Defense.