A woman is aided by first responders after sustaining injury on a bike path in lower Manhattan in New York, NY, U.S., October 31, 2017. Brendan McDermid | Reuters

The "caliphate" may be in ruins, but that doesn't mean ISIS is gone forever. Terror attacks are likely to increase in 2018, as the destruction of the Islamic State's physical stronghold in Iraq and Syria will strengthen its will to strike out abroad, experts say.



"ISIS will want to show that they are still in the fight, and their followers remain as fanatical as ever," said Lewis-Sage Passant, a former British Army intelligence officer and founder of travel security company HowSafeIsMyTrip. "The number of attacks globally will likely increase as the group switches focus from the war in the Middle East to international terrorism." Adam Deen, executive director of counter-extremism think tank Quilliam, makes a similar argument. "We're going to see without a doubt more attacks in the West," he told UK newspaper the Independent in October. Deen said ISIS is now more focused on revenge, and warned against the false sense of victory that many expressed after the death of Osama Bin Laden.

They will be less able to mount well-funded operations such as the Paris attacks. But they want to show that they are still in the fight. Lewis-Sage Passant former intelligence officer, speaking on ISIS

Propaganda will continue online and elsewhere, despite the destruction of its main source in the former ISIS "capital" of Raqqa, Syria.



"Threats in the West will persist in the form of people who are still inspired by the propaganda that has been disseminated by ISIS," Anthony Richards, an assistant professor in Terrorism Studies at the University of East London, told CNBC.



"I think in the longer term, the defeat of ISIS and the propaganda defeat that goes with that will actually reduce the threat in the UK and Europe, but ... In the shorter term, we'll still see more terrorist attacks."



Some foresee more frequent attacks, accompanied by a decrease in lethality, including a rising number of knife and vehicle attacks. "The loss of ISIS' central coordination and revenue-generating capabilities means that they will be less able to mount well-funded operations such as the Paris attacks," Sage-Passant said, "But they want to show that they are still in the fight."

5,600 fighters go back 'home'

An estimated 25,000 foreign fighters from more than 100 countries left their homes to fight in Syria. A report by the Soufan Center in October put the current figure for returned fighters at 5,600 from 33 countries, and revealed that on average 20 to 30 percent of those from Europe are already back. In the U.K., Sweden and Denmark, a whopping 50 percent have returned. Almost 20,000 names have been shared with Interpol.



For the U.S., the figure is far smaller: The Soufan Center reported 129 Americans made it to the battlefields of Syria or Iraq, and only seven of them have returned as of October. Seventy-seven out of 135 people charged with ISIS-linked terrorism offenses have been convicted as of August.

That presents a tremendous challenge for domestic authorities. However, many warn that blanket bans on movement or entry, such as U.S. President Donald Trump's travel ban on six majority-Muslim countries, aren't likely to effectively stem the threat.



"The underlying ideology that drives the violence of groups like ISIS is not a physical entity that can be stopped by territorial borders," Mubaraz Ahmed, analyst at the Tony Blair Institute for Global Change, told CNBC. Domestic networks and individuals are what matters, he argued — after all, ISIS attacks in Europe have been carried out almost exclusively by citizens or residents of those countries.

The return of Al-Qaeda