(CNN) -- The Sunni Muslim extremist group ISIS has torn through eastern Syria and northern and western Iraq in recent months, seizing territory for what it says is its new Islamic caliphate and butchering those who stand in their way.
The situation has become especially dire for minority groups. Iraqi Christians are on the run, with ISIS taking over their largest city, Qaraqosh, and giving them an ultimatum: Convert to Islam or die. In addition, thousands of families from the Yazidi minority fled from ISIS and became trapped on a mountain in northern Iraq without food, water or medical care.
U.S. President Barack Obama said Thursday that he has authorized targeted airstrikes in Iraq, if deemed necessary, to help Iraqi forces protect the trapped Yazidi, and to protect American personnel working with the Iraqi military in Irbil, the largest city in Iraq's Kurdish region.
While ISIS, which calls itself the Islamic State but formerly was known as the Islamic State in Iraq and Syria, operates in the Middle East, U.S. Attorney General Eric Holder has expressed 'extreme concern' that its violence could spread to Western shores. Its members may be mingling with Yemeni bomb makers who have a track record of getting devices on Western planes, and thousands of Europeans have gone to Syria ready to give up their lives.
'It's more frightening than anything I think I've seen as attorney general,' he told ABC News earlier this year.
Canadian jihadi video recruits for ISIS Stopping the next domestic terror attack Bashar al-Assad sworn in for 7-year term Refugees risk all to escape ISISHere are five reasons why the fight in Syria and Iraq could spill over to the West.
1. ISIS has the manpower, money and know-how to hit the West, if it decides toThe nightmare scenario is that ISIS leaders or other terrorist groups in Iraq and Syria launch attacks in the West. They are well-positioned to unleash such carnage if they choose. Many of at least 2,000 European militants who have traveled to Syria joined ISIS. That has given groups the opportunity to train them and send them back home to launch attacks. A number have crossed into Iraq.
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. About 100 Americans also have traveled to fight in Syria -- one carried out a suicide bombing in May.
So far, though, ISIS and its fierce rival, the Syrian al Qaeda affiliate Jabhat al-Nusra, have not seen attacking the West as anything near a priority.
Their focus instead has been on fighting the regime of Syrian President Bashar al-Assad and expanding their zone of operations in the region. 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.
ISIS -- a group 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. 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.
Iraqi Shiite volunteers who have joined government forces to fight the militant group ISIS take part in a training session near Basra, Iraq, on Thursday, August 7. ISIS -- known for killing dozens of people at a time and carrying out public executions, crucifixions and other acts -- has taken over large swaths of northern and western Iraq as it seeks to create an Islamic state that stretches from Syria into Iraq.Photos: Iraq under siegeIn 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 2 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.
If the United States launches strikes to weaken ISIS, the group could strike back at the West, financing attacks with the tens of millions of dollars in its cash reserves. Last month, its supporters launched a Twitter campaign -- #CalamityWillBefallUS -- warning of such attacks.
But, 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.
It's a Catch-22 that worries U.S. officials.
'There's going to be a diaspora out of Syria at some point, and we are determined not to let lines be drawn from Syria today to a future 9/11,' FBI Director James Comey warned in May.
Iraq conflict creating strange alliances Will U.S. take direct role in Mideast? Iraqi family counts cost of ISIS conflict 2. Expert Yemeni bomb makers may be mingling with like-minded Westerners in SyriaCoping with chaos in BaghdadU.S. officials worry about Yemeni bomb makers who are skilled in making explosive devices that are difficult to detect at airport security. The fear is that they are sharing their knowledge with terrorist groups in Syria with significant numbers of Western passport holders in their ranks.
Holder called it a potentially 'deadly combination.'
Ibrahim al Asiri, the ingenious chief bomb maker for al Qaeda in the Arabia Peninsula, a Yemeni al Qaeda affiliate, is thought to have trained a cadre of apprentices. Early this year, U.S. officials became worried that some may have traveled to Syria. Hundreds of Yemenis have traveled to fight there, and officials worry that AQAP and Jabhat al-Nusra are building ties. With ISIS dominating headlines and winning the battle for new recruits, it is possible that al Qaeda affiliates may try to restore their relevance by planning a spectacular attack.
3. Western fighters who leave Syria could lash out back homeAl Asiri built the 'underwear' device that a recruit partially detonated on a plane over Detroit on Christmas Day in 2009. Since then, he has built increasingly sophisticated devices and has experimented with new designs for a shoe bomb. But AQAP has recruited relatively few Westerners into its ranks, limiting -- at least until now -- its bomb makers' ability to target Western aviation.
But there is concern that al Asiri's knowledge is spreading more widely. This month, the U.S. State Department said a Norwegian convert, Anders Dale, had received extensive instruction in explosives after joining AQAP in Yemen. It was not made clear whether this training was provided personally by al Asiri. Nor was it said where the Norwegian is now thought to be.
Photographs of victims of the Bashar al-Assad regime are displayed as a Syrian Army defector known as 'Caesar,' center, appears in disguise to speak before the House Foreign Affairs Committee in Washington. The briefing on Thursday, July 31, was called 'Assad's Killing Machine Exposed: Implications for U.S. Policy.' Caesar was apparently a witness to al-Assad's brutality and has smuggled more than 50,000 photographs depicting the torture and execution of more than 10,000 dissidents. Photos: Syrian civil war in 2014What most keeps European counterterrorism officials awake at night is the potential threat from hundreds of extremists who have returned home after fighting with terrorist groups in Syria. While little evidence has emerged so far that ISIS or Jabhat al-Nusra 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, has been charged with gunning down four people at a Jewish museum in Brussels, Belgium, in May. When he was arrested, police discovered a Kalashnikov in his possession wrapped in a flag with ISIS insignia. After the attack, an ISIS fighter said on social media that Nemmouche had joined the group, but ISIS itself did not claim responsibility, suggesting to investigators that he planned the attack himself.
'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 last month. 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 catch-up.
4. Would-be jihadists could become radicalized 'lone wolves''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.
5. Foreign fighters who go home could build terror networks of their ownThe 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.
Officials worry that anger over 'trigger events' such as future U.S. strikes in Iraq or the arrests of fighters returning from Syria could result in lone-wolf attacks. Anger about events in Gaza could be another.
Around 7,000 foreign fighters have traveled to fight in Syria, many from the Arab world. This could see 'blowback' across the region as fighters return to their home countries and build up jihadist terrorist groups. Like in Afghanistan two decades ago, fighters are building personal relationships in this melting pot that will form the basis of the transnational terrorist networks of the future. For example, a battalion of hardened jihadists from eastern Libya is fighting alongside ISIS in Syria while Egyptian ISIS recruits have returned to the Sinai, bolstering militant groups there.
Now that al Qaeda has largely switched its operations from the Afghanistan-Pakistan border region to the Arab world, officials fear that its various affiliates in the region will increasingly coordinate and pool resources, creating a significant long-term security threat on the doorstep of Europe.
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