Most consequential is the war in Yemen. In 2015, a Saudi-Emirati coalition intervened to prevent the Houthis, an Iran-allied rebel faction, from gaining control of the country. But those forces have yet to dislodge the Houthis from any of the major cities in northern Yemen, and in the meantime the country’s situation overall has become the world’s worst humanitarian crisis, according to the United Nations. The continued chaos offers Iran a low-cost opportunity to drain Saudi resources, and it opens up new possibilities for Al Qaeda and the Islamic State to take root in Yemen.

M.B.S.’s overly ambitious and misguided foreign policy initiatives have left Saudi Arabia weaker and the region less stable, undermining American objectives. The same goes for Mr. Khashoggi’s disappearance. He was no real threat to M.B.S., but now his suspected abduction and murder by Saudi officials in Istanbul is worsening Saudi Arabia’s relations with Turkey, a major regional power, at a time when the two countries should be looking for common ground to limit Iran’s clout in Syria.

Yet Mr. Trump has made no effort to restrain M.B.S.’s adventurism. The two men’s apparent closeness is not the cause of the prince’s recklessness; for example, Saudi Arabia intervened in Yemen while Barack Obama was still president. But the Trump administration’s full-throated endorsement of M.B.S. is hurting America’s interests.