The United States may have been calling the shots in Tuesday's air strikes against Islamic State terrorists, but in a historic shift, five Arab monarchies – Saudi Arabia, United Arab Emirates, Qatar, Bahrain and Jordan – joined the war to fight radical Muslim extremists, potentially scrambling Arab politics forever.

Scores of Islamic State fighters were killed in a U.S.-led aerial operation carried out for the first time on Syrian soil. In a separate U.S.-only action, dozens of other militants belonging to an al-Qaeda-linked group planning terror attacks in the West were also killed.

U.S. President Barack Obama praised the participation of Arab allies in the campaign against the Sunni extremists. "America is proud to stand shoulder to shoulder with these nations on behalf of our common security," he said.

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"The strength of this coalition makes clear to the world that this is not just America's fight alone."

It's been 23 years since a U.S.-led coalition has had so many Arab partners. In 1991, the enemy was Iraqi leader Saddam Hussein, whose army had overrun the small Arab state of Kuwait. Today, the group that calls itself Islamic State is the adversary, having captured large swaths of eastern Syria and western Iraq and threatened the region.

"These Arab monarchs obviously feel that [the Islamic State] will be coming after them eventually, so better to act now, together," said Michael Knights, an expert of Gulf states at the Washington Institute.

They have plunged into using a military response to a situation where in the past they would have preferred to tie themselves into diplomatic knots.

The Gulf Cooperation Council, to which all the Arab partners except Jordan belong, "has gained an appetite for preventive action," Mr. Knights said. "They've been preparing for this day for a long time."

Indeed, these countries have had so many training exercises with U.S. forces that the attack on the Islamic State "must have felt like another exercise, only with real bombs," Mr. Knights added.

In joining the U.S.-led operation, these countries "are trying to balance Iran's power in Iraq," said Yezid Sayigh, a military analyst with the Carnegie Center's Middle East office in Beirut. "They want to show that they can step up to the plate and that Iran isn't needed to win this thing."

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The participation in the air raids of the ultra-conservative Saudi Arabia is the biggest surprise for many people.

Crown Prince Salman bin Abdulaziz, the heir apparent and Saudi Defence Minister, explained in a National Day speech on Tuesday that his country needed to be more assertive in combating militancy.

"We are concerned because we have not done enough to protect our nation from extremism, and its youths from militancy and radicalism," Prince Salman said. This has led some "to adopt violence and replace the doctrine of tolerance with that of takfir [declaring someone to be an infidel]."

Joining a U.S.-led initiative is not without risk and Saudi Arabia was specifically targeted in the most recent audio statement by the Islamic State, released on social media late Sunday. In it, the Saudi monarchy, known as the Custodian of the Two Holy Mosques (Mecca and Medina), is described by IS spokesman Abu Mohammed al-Adnani as merely "guard dogs for the Jews and a stick in the hands of the crusaders."

For its part, Qatar is clearly uncomfortable in the coalition, said Mr. Sayigh, as shown by its taking the most minor of all roles in Tuesday's operation against IS strongholds in northeastern Syria.

Qatar and the United Arab Emirates already are at odds, he said, noting that the two are engaged in a "virtual proxy war in Libya," where Qatar is backing one, more radical, political faction and the Emirates, a more moderate one. In Syria, Mr. Sayigh explained, "Qatar has favoured more extreme rebel forces, while the Emirates and Saudi have sought out moderates."

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Qatar is ill at ease with these other two Arab states, he said, and "this coalition could come apart at the seams very quickly – I give it two or three weeks."

This unease is particularly acute because of the other target on Tuesday: the Khorasan forces in northwestern Syria. Khorasan, a division of al-Qaeda, is working in Syria under the umbrella of the rebel group known as the Nusra Front. It was attacked solely by U.S. forces and was not among those targets attacked by their Arab allies.

The attack on Khorasan, however, turned out to be more of an attack on the Nusra Front, said Mr. Sayigh, something that must have troubled Qatar greatly.

"I think this was a big mistake by the U.S.," the Carnegie analyst said. "Nusra controls the border in that area and it's a vital link for many other rebel groups, including the very moderates the U.S. says it wants on its side" to take the fight to the Islamic State.

"This attack could really mess things up," he said.

The participation of the five Arab states in Tuesday's operation did pay at least one early dividend – it helped keep Syria from criticizing the air raids on its soil.

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While there was little Syria could have done to stop a U.S. attack, and would have looked weak if it merely denounced it, President Bashar al-Assad appeared to enjoy seeing so many of his Arab antagonists take the fight to his deadliest enemies.

"The Syrian Arab Republic stands with any international effort to fight terrorism, no matter what a group is called – whether Daesh or Nusra Front or something else," the Assad regime said, using a pejorative Arabic name for the Islamic State.