SAUDI ARABIA and its de facto ruler, Crown Prince Mohammed bin Salman, are commonly and justly blamed for a disastrous military intervention in Yemen that, while failing in its objective of defeating Iranian-backed forces, has killed thousands of civilians and triggered the world’s worst humanitarian crisis. But an equal partner in the fiasco has been the United Arab Emirates, which deployed thousands of troops to Yemen and launched a siege of the port city of Hodeidah that threatened the country’s food supply at a time when millions of people were at risk of starvation.

Now, the UAE has reportedly pulled its forces out of the Hodeidah area and drastically reduced its military commitment in Yemen. It’s a long-overdue but welcome step that could contribute to the negotiated peace settlement that is the only way to end the civil war and humanitarian crisis.

The UAE decision appears to have been motivated in part by the growing backlash against the war in Congress, which has voted on multiple occasions to suspend U.S. support for the Saudi-led coalition or to block arms sales to the two Persian Gulf countries. President Trump has vetoed one resolution and is expected to veto the others so far passed. But, last week, the House added an amendment to the annual defense authorization bill suspending air-to-ground arms sales to Saudi Arabia and the UAE for one year; if the measure emerges intact from a House-Senate conference committee, the White House could be forced to accept it.

A second reason for the UAE pullback is the mounting tension between the United States and Iran, caused by Mr. Trump’s decision to reapply crippling sanctions. Concern that the Persian Gulf states could be a target for Iranian retaliation in the event of a military conflict seems to have motivated a decision to return Patriot anti-missile batteries and other defensive systems to the UAE from Yemen. In a briefing for reporters last week, a UAE official appeared to confirm that calculation, saying the UAE was not blind to “the overall geostrategic picture.”

Unfortunately, those same tensions may impede the settlement that Yemen desperately needs. Having unnecessarily initiated a larger crisis with Iran, the Trump administration can hardly expect Tehran to lean on its Yemeni allies, known as the Houthis, to make peace with the Saudi-backed government. On the contrary, the Houthis have been firing missiles at Saudi airports in recent weeks. Mohammed bin Salman similarly shows no sign of backing away from the war, even though the UAE withdrawal has made victory an even more remote prospect.

An enduring settlement in Yemen will almost certainly require a de-escalation of tensions across the region — which, in turn, can come about only if the United States and Iran reach some kind of detente. For now, Mr. Trump appears bent on continuing his “maximum pressure” campaign against Iran while supplying Saudi Arabia with fresh bombs for Yemen. So Congress could play a constructive role by ratifying the ban on those deliveries — and by sending the firm message that war with Iran is not an option.

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