Riyadh, Saudi Arabia: As Saudi Arabia’s crown prince, Mohammed bin Salman, rose to power a decade ago, officials and pundits in Washington sometimes compared him to the de facto ruler of the neighbouring United Arab Emirates, Sheikh Mohammed bin Zayed Al Nahyan.

Both royals presented themselves as authoritarian reformers. In 2015, they teamed up in Yemen, launching a disastrous military intervention to beat back Iran-supported rebels, and later joined forces in a bitter political dispute against their neighbour Qatar.

But today, the two men are increasingly at odds, leading their oil-rich Persian Gulf countries into confrontation and competition with each other in the Middle East, Africa and beyond.

Saudi Crown Prince Mohammed bin Salman, left, and the UAE’s Sheikh Mohammed bin Zayed Al Nahyan in 2021.Credit: AP

On Tuesday, long-simmering tensions burst into the open as a Saudi-led airstrike targeted a UAE shipment as it arrived in Yemen, with the accusation that the UAE was sending weapons to a separatist group.

It was the dramatic climax of a spat between the one-time partners, who now support opposing groups in Yemen and Sudan, have pursued differing oil policies and acted against each other in economic and trade matters.

Because of the vast global influence of Saudi Arabia and the UAE – as major energy exporters, with enormous sovereign wealth and diplomatic sway – the rift has the potential to move markets, derail investments and disrupt sensitive negotiations around the world.

This frame grab from video broadcast by Saudi state television on December 30 shows what the kingdom describes as a shipment of weapons and armoured vehicles coming from the UAE, at Mukalla, Yemen. Credit: AP

For Washington, the rupture has the makings of a serious headache. US Secretary of State Marco Rubio spent Tuesday working the phones with his Saudi and UAE counterparts.

Both countries have made extravagant promises to pour money into the United States, and they are key players from whom US President Donald Trump hopes to win support for his Middle East policies, including in Israel and the Gaza Strip.

“No one in the Gulf wants to see another Gulf crisis, so everyone has an interest in containing this,” said Yasmine Farouk, the director of the Gulf and Arabian Peninsula project for the International Crisis Group.

Still, a fundamental schism remains.

In his early years, Crown Prince Mohammed........

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