At the height of their power they once controlled an area roughly the size of Britain.

In all, some 10 million people fell under the brutal rule of the Islamic State (IS) group, who shocked the world with their massacres, beheadings and systematic sexual enslavement of women. 

Also known in the Arab world as Daesh and by the acronyms ISIS and ISIL, for years the jihadists of IS were the terrorist headline makers.  

So what has become of IS and what threat, if any, do they pose now? 

It was earlier this year, in the now devastated Syrian city of Raqqa, once the self-proclaimed capital of their Islamic caliphate, that I first got a sense of what IS have now become.

“It’s best that you leave the city and are back on your way out by mid afternoon at the latest, certainly before dark,” a young Kurdish officer called Farman told me, as we talked in the dilapidated stairwell of an apartment block in a central Raqqa neighbourhood last May. 

Farman and his fellow Kurdish fighters, as part of the Syrian Defence Forces (SDF) that had ousted IS from the city, had been billeted for months in this block of flats stinking of raw sewerage and littered with spent cartridge cases.

“Daesh sleeper cells and supporters still cause problems, but we have them contained,” Farman assured me, while admitting that IS continued to have a presence in the city.

Farman’s concerns were more widely endorsed recently by Kino Gabriel, a spokesperson for the SDF, a key Western ally fighting IS, who warned that the group was taking advantage of persistent political instability in Syria and Iraq. 

“Our estimation of IS’s power was wrong. We have realised that there are more IS fighters than we thought,” Mr Gabriel said in an interview earlier this week.

It’s not just the number of remaining IS fighters that are a source of worry but also the way they are making their comeback.

Not only in Raqqa but in other parts of Syria and in neighbouring Iraq, IS have returned to their insurgent roots and re-emerged as a guerrilla force carrying out a rising number of kidnappings, killings and bombings. 

Counterterrorism officials speak of an “atomised, clandestine network of cells with a decentralised chain of command.”

According to a recent US Department of Defence report an “effective, clandestine ISIS organisation appears to be taking hold” almost four years after the group controlled swaths of Syria and as much as a third of Iraq.

For the third straight year, deaths from terrorism declined in 2017.

Of the 18,814 caused by terrorists around the world, well over half were due to the actions of just four groups, of which IS was the most deadly. 

The other groups were the Taliban in Afghanistan and Pakistan, al-Shabaab in Somalia and Boko Haram in north-eastern Nigeria

The figures compiled by the Institute for Economics & Peace (IEP) as part of the Global Terrorism Index 2018, found that these four organisations were responsible for 10,632 deaths in 2017, with IS accounting for most of these. 

In both Syria and Iraq security forces are struggling to adapt to a shift in tactics that sees IS adopt a back to the future approach by again becoming a clandestine insurgency network, just as it was originally in its earliest form back in 2002-2003. 

Much like these predecessor organisations, IS now are seizing on local grievances, taking advantage of ungoverned space by operating from cell structures to conduct hit and run attacks, kidnappings for ransom, targeted assassinations, and bombings using improvised explosive devices (IED’s).

“While IS territorial control has been reduced to minor pockets of rural Iraq, the group was still carrying out an average of 75 attacks per month in 2018, including a doubling of attacks year over year in Kirkuk province,” a recent report by the Centre for Strategic and international Studies (CSIS) concluded. 

Hashem al-Hashemi, an Iraqi government adviser, has compared IS’s current strategy to that of Osama bin Laden’s al-Qaeda forces who, back in 2001, though besieged and holed up in Afghanistan’s Tora Bora, mountain and desert areas, were still capable of devastating attacks.

Just as IS have lost territory, so too have they lost some of their vast income from oilfields in Iraq and Syria that once helped it become the world’s richest terrorist group.

But as Rukimini Callimaci, a correspondent with The New York Times and an acknowledged expert on IS recently pointed out, the jihadists are also adopting a back to the future approach to their fundraising.

“I was just in Baghdad a couple of weeks ago, and I met with coalition officials and unfortunately the number that they were citing for how much money ISIS still has on hand, just in Iraq and Syria, is over $300 million (£235m). So that gives you a sense of just how wealthy this terrorist group is,” says Ms Callimaci.

Today IS, as it resurrects itself, earns the bulk of its revenue from extortion and front companies, including car dealerships to currency exchanges.

“They are now going back to the types of fundraising that they were doing before, namely illegal taxation in areas that they do not control, coming into areas and telling businesses if you don’t pay up, like the mafia, if you don’t pay you’re going to face the consequences,” Ms Callimaci said.

“This is a group that has known how to finance itself without territory for over a decade. So I don’t expect them to go bankrupt any time soon,” she added.

As if all this were not disquieting enough, IS - as it hunkers down in caves and fortified tunnels in the small city of Hajin in eastern Syria and a few surrounding villages in the Euphrates River Valley - wields a diminished but still formidable social media prowess to rally its followers on the ground and on the internet.

The jihadists commitment to overseas attacks was reiterated again this week when IS supporters online invoked the 2016 Berlin Christmas market attack and vowing “retribution” to come. 

It was yet another chilling reminder that, while IS might be down, it is far from out of the global terror business. Unpalatable as the thought is, they most likely will be back making headlines in the very near future.