In the years after he was named the deputy crown prince of Saudi Arabia, in 2015, Mohammed bin Salman—known informally as M.B.S.—has accumulated remarkable power in the country. (He became crown prince in 2017.) Saudi Arabia, under his de-facto leadership, embarked on a military campaign against the Houthis in Yemen, temporarily kidnapped the Prime Minister of Lebanon, and blockaded Qatar. Many of these actions were part of a campaign to isolate Iran, whose Supreme Leader, Ayatollah Ali Khamenei, M.B.S. referred to in 2017 as “the new Hitler.” While conducting his aggressive foreign policy, M.B.S. had a close ally: Mohamed bin Zayed, the President of the United Arab Emirates. “M.B.Z. saw M.B.S. as a younger version of himself: smart, energetic, and eager to confront enemies,” Dexter Filkins reported for this magazine in 2018.

In the past few months, however, things have changed. The alliance that promised to reshape the Middle East has collapsed into acrimony, with Saudi Arabia and the U.A.E. on different sides of violent conflicts in Yemen and in Sudan. The two countries are increasingly competing for economic opportunities in the region, while Saudi Arabia sees the U.A.E. as too willing to ally with Israel, and the U.A.E. seems resentful of Saudi Arabia’s power.

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Why has a falling out occurred over the past several months?

The trigger was in early December when the U.A.E.-backed forces in Yemen, especially the separatist Southern Transitional Council, moved into two provinces of eastern Yemen and upended the fragile balance of power in Yemen. The Saudis saw this as highly provocative, as unhelpful to the anti-Houthi coalition in Yemen, and as a threat, because of the Yemeni-Saudi border, to Saudi security. And the U.A.E.’s initial advance was also on the same day Gulf leaders were meeting in Bahrain. So, in Riyadh, it was seen as a major provocation that a close ally, such as the U.A.E., would green-light an advance that was seen as so antithetical to Saudi interests.