It is more than 23 years since Arab countries last made common cause to join U.S.-led military action, and it has taken the threat of Islamic State to persuade them that any public backlash in an already turbulent region is a price worth paying.
Of the five Arab states named by Washington as supporting U.S.-led strikes against the jihadist group in Syria, Bahrain, Jordan and the United Arab Emirates (UAE) confirmed they had actually flown sorties. Saudi Arabia said it had "participated in military operations", and Qatar was believed to have offered only logistical or political support.
But association with the attacks, after years of U.S.-led wars that have antagonized Muslims around the world, is a risk these states are ready to run to quash a group that promises to refashion the Middle East as an Islamic caliphate.
"We see Islamic State as an existential threat. If we don't put a stop to it, it will expand into our area," said Sami al-Faraj, an adviser to the Gulf Cooperation Council, which groups Saudi Arabia, Bahrain, UAE, Kuwait, Oman and Qatar.
The air and missile strikes were designed to undermine the military and organizational prowess of a group that controls large parts of northeast Syria and has seized swathes of Iraq since June.
As it sucks in support from other less successful rebel groups, and sophisticated American weaponry from routed Iraqi forces, Islamic State has made clear it seeks nothing less than to place the entire Muslim world under medieval-style theocratic rule - a message abhorrent to the Gulf's dynastic rulers.
Not since a multinational coalition ejected Iraq from Kuwait in 1991 have such a large number of Arab countries aligned themselves publicly with U.S.-led armed action in the Arab world.
"They are no longer active, influential neutrals, and are now fast becoming frontline states in a war that is likely to engulf the whole region," said British academic Christopher Davidson.
NEW CONFIDENCE
Although the Gulf states are heavy purchasers of Western military hardware, and Qatar and UAE lent military support to the Libyan forces that toppled Muammar Gaddafi with Western help in 2011, they are normally seen as reliant on the West for their defense in any emergency, as was the case with Kuwait.
The disclosure of Saudi Arabia's involvement in U.S.-led attacks also breaks with the conservative Sunni Muslim kingdom's traditional aversion to any publicity, let alone on such a sensitive issue.
"The impression is that the Saudis are willing to trumpet their role," said Neil Partrick, an associate fellow at the Royal United Services Institute in London. "That's a striking development, even though we don't know what their role is."
The kingdom's heir apparent and defense minister, Crown Prince Salman, said in a national day address carried by Saudi media on Tuesday that Riyadh needed to counter militancy more assertively.
"We are concerned because we have not done enough to protect our nation from extremism, and its youths from militancy and radicalism, leading some to adopt violence and replace the doctrine of tolerance with that of takfir (declaring to be an infidel)," local newspapers reported.
GCC adviser Faraj said Gulf Arab action now was "a message to the militants, and to the radicals among our own people, that we mean business about stopping the Islamic State".
The UAE also appears ready not only to take action but also to present a more confident and assertive front.
Ebtesam Al-Ketbi, chairwoman of the Emirates Policy Center in Abu Dhabi, told Reuters the UAE's support was in line with its growing capabilities. "It has become a military, economic and political power to reckon with," she said.
Asked about the risk of inviting attacks on the UAE, Ketbi said "terrorism" was a possibility, whether the UAE took part or not.
"There is nothing without a risk. But you are part of an international alliance and you cannot be inactive in the face of terrorism when you have all these resources," she told Reuters.
"GUARD DOGS FOR THE JEWS"
Islamic State itself has threatened to attack any state that supports the U.S. campaign.
Its spokesman Abu Muhammad al-Adnani on Monday reiterated a threat to Saudi Arabia, and dismissed its rulers as "guard dogs for the Jews, and a stick in the hands of the crusaders to be used against Islam".
Riyadh has two big worries about Islamic State: that it will consolidate its rule in Iraq, creating a de facto haven for militants along the kingdom's northern border; and that it will encourage radicals inside the kingdom to mount their own raids.
Faraj said the level of alertness of security forces in Gulf Arab states had been raised for weeks and "today it is at the highest" because of the U.S.-led attacks.
He said security would be heightened in particular near air bases and at logistics and fuelling facilities.
But security is not the only kind of worry facing the Arab members of the anti-Islamic State alliance.
Maintaining a united front may also be hard, because of disputes between Qatar and some of its Gulf Arab neighbors about the role of Islamists in Arab politics.
Although the Gulf states are all opposed to Syrian President Bashar al-Assad, Saudi Arabia last year leaned heavily on Qatar to desist from providing money and weaponry to some of Syria's more radical Islamist rebels.
Qatar is therefore likely to have been unhappy about the raids' targeting of the Nusrah Front, a group linked to al Qaeda that it seems to see as an authentic Syrian opposition group, diplomats said.
A source close to the Qatar government told Reuters the overnight attacks would not solve anything. He said it was unfair to target only Islamic State when Assad "has been left to kill his people for years".
