(CNN) -- The U.S. air strikes carried out Friday against ISIS are likely to have one unintended consequence: immediately increase the terrorist threat in Europe and the United States, Western counter-terrorism officials tell CNN.
Within hours of U.S. military jets and drones conducting a strike on ISIS artillery that had been used against Kurdish forces defending Irbil, ISIS supporters called for retaliatory attacks against the United States.
"It is a clear message that the war is against Islam and the mujahideen. The mujahideen must strive and seek to execute proactive operations in their own home, America, to discipline America and its criminal soldiers," Abu al-Ayna al-Khorasani, an administrator of Shumukh al-Islam, the top-tier forum for ISIS propaganda, wrote on his account Friday, according to a translation by the SITE Intelligence group.
Other ISIS supporters railed against the United States using the Twitter handle #AmessagefromISIStoUS, posting images of the wreckage of the twin towers. "Don't forget 11 Sept .. Maybe US citizens want more like that," one extremist tweeted. In June after ISIS captured Mosul, its supporters had warned against strikes in a Twitter campaign #CalamityWillBefallUS.
Here are three ways ISIS and its supporters could retaliate for U.S. military action against its fighters:
1. ISIS could unleash a crash program to attack the West
The nightmare scenario is that ISIS leaders initiate a crash program to launch attacks in the West. They are well-positioned to unleash such carnage if they choose.
ISIS, which calls itself the Islamic State but formerly was known as the Islamic State in Iraq and Syria, is thought to have tens of millions of dollars in cash reserves and it operates training camps on a scale last seen in Taliban-run Afghanistan. The 9/11 operation, by point of reference, cost $500,000.
European officials tell CNN up to 1,000 European extremists are believed to have joined ISIS. Dozens of Americans are probably fighting with the group, too. This gives ISIS the opportunity to train them and send them back home to launch attacks.
These European fighters also could pose a threat to the United States because many Europeans do not need a visa to enter the U.S.
There also are fears these Western fighters might be equipped with sophisticated devices that could evade airport security. Early this year, U.S. officials became worried that some AQAP bomb-makers trained by master-bombmaker Ibrahim al Asiri may have traveled to Syria.
This combination of threats was described by U.S. Attorney General Eric Holder as "more frightening than anything I think I've seen as attorney general," in an interview with ABC News several weeks ago.
So far, though, ISIS has not seen attacking the West as anything near a priority.
Its focus instead has been on expanding the territory it controls in Syria and Iraq.
The political turmoil brought about by the Arab Spring has made the ultimate dream of global jihadists -- the adoption of their kind of Islamic rule across the Arab world -- seem tantalizingly close. Attacking the West, which for al Qaeda leaders was always a means to this end, has become something of a sideshow.
It is possible that the limited aerial campaign initiated by U.S. President Barack Obama will not see a significant switch of focus from ISIS towards attacking the West. The White House's stated objectives do not include degrading the group.
ISIS -- previously known as al Qaeda in Iraq -- has never prioritized targeting Western soil, instead preferring to focus on fighting "infidels" at home.
In the decade since the Jordanian terrorist Abu Musab al-Zarqawi created the group, it has not been directly behind any plot on Western soil, even during the years the United States military engaged it in intense counter-insurgency operations across Iraq.
By contrast, in the decade after the September 11 terrorist attacks, al Qaeda operatives in Pakistani tribal areas provided wave after wave of Western recruits training on how to make bombs out of chemicals and components readily available in home-improvement and beauty-supply stores in the West.
To date, only one suspected ISIS recruit who has returned to Europe is alleged to have built such a device.
In February, French police arrested a man they identified as Ibrahim B., a 23-year-old French-Algerian, and retrieved three soda cans filled with nearly a kilogram (about two pounds) of the high explosive TATP from his Cannes apartment. French police suspect that in the 18 months he fought in Syria, he learned how to make TATP, an unstable and difficult-to-transport high explosive used to build detonators in multiple al Qaeda plots against the West.
It is not clear whether ISIS signed off on his alleged plot.
While some Western recruits are taught how to make improvised explosive devices in Syria, there is little indication yet that the group has created a training program tailored to attacking the West. The worry is that that could change. After a decade of insurgency in Iraq, no other group has more expertise in making IEDs.
2. Returned fighters could take matters into their own hands
The most immediate threat to the West comes from hundreds of extremists who have returned home after fighting with terrorist groups in Syria. While little evidence has emerged so far that ISIS has directed them to launch attacks, their urban warfare skills would make them especially dangerous.
The first terrorist attack on Western soil linked to Syria probably followed this trajectory. Mehdi Nemmouche, a French-Algerian who spent a year in Syria and was recruited into ISIS, has been charged with gunning down four people at a Jewish museum in Brussels, Belgium, in May. ISIS itself did not claim responsibility for the attack, suggesting to investigators that Nemmouche planned the attack himself.
Among Western countries Europe has by far the greatest number of returnees.
"The threat of attacks has never been greater -- not at the time of 9/11, not after the war in Iraq -- never," a European counterterrorism official told CNN in June. He envisaged a flood of small-scale but effective and chilling attacks similar to the Brussels shooting.
European counterterrorism officials are worried the gains made by ISIS in Iraq will lead to a surge of travel to the region. In identifying who has traveled, they are often playing catchup.
"In most cases, we know within two weeks a guy has gone to Syria. But 10%-15% of the time, it can be several months before we figure it out. Inevitably, there will be some we have no idea about," one official told CNN.
But even those they know about are difficult to track. Nemmouche was on a watch list when he returned to Europe. European officials tell CNN it is impossible, because of the cost, to conduct 24-hour surveillance on any but a small fraction of people who have returned from Syria.
3. 'Lone wolves' could lash out
The Boston Marathon bombings illustrated the danger posed by extremists learning bomb-making skills over the Internet without having to travel to jihadist encampments overseas.
European officials say anger about events in Syria and Iraq and excitement about the gains made by ISIS have spiked radicalization to unprecedented levels across the continent.
Though the animus is not directed as squarely against the West as it was during the Iraq war, ISIS' viscerally anti-Western ideology is attracting a growing following in extremist circles in Europe. It matters little that European countries are not currently involved in air strikes: ISIS supporters believe all Western countries are working together to attack Islam.
The Costs of inaction
The likely increased terrorist threat in the West that will arise in the coming weeks and months from U.S. air strikes in Iraq should be weighed against the risks of letting ISIS grow unchecked.
If ISIS is able to consolidate its territorial gains, it could set up training camps to rival any run by al Qaeda in Afghanistan before 9/11. Though its focus may be in Iraq and Syria for now, its viscerally anti-Western ideology means it would be foolish to discount it as a major potential future threat.
Though the unfolding humanitarian crisis in Iraq may have left the Obama administration little choice but to act, the concern is the limited US aerial campaign has now stirred the hornet's nest without doing anything to decisively degrade ISIS.
And there could be one other unintended consequence. A rally-round-the-flag effect may now strengthen the ISIS position in the global jihadist movement, leading to even higher levels of fundraising and recruitment.
In recent months the group has faced fierce criticism from al Qaeda and others for its declaration of an Islamic caliphate and its brutal methods. But with the budding Islamic State now being bombarded by the "Crusaders," such criticisms may fade, at least for a time.