The two organisations also share a lineage. The threadbare remnants of AQI – all but crushed by the US troop surge in Iraq of 2007 and the “sons of Iraq” movement to turn Sunni tribes against the jihadis – morphed into the earliest version of Isis.

But more importantly, Isis is also the operational, strategic and ideological twin of its predecessor.

“There is almost no difference in the organisations,” says Afzal Ashraf, a former RAF captain in Iraq and now consultant at the Royal United Services Institute.

Mr Ashraf points in particular to the shared heritage of Isis and AQI in drawing on former members of Saddam Hussein’s Ba’athist regime. Both are “parasitic insurgencies” that co-opt disenfranchised factions to their cause, he says.

It is perhaps for this reason that both AQI and Isis have historically shared a primary concern with the “near enemy” – other Arabs – rather than the far enemy – Western infidels – as their main targets. Isis, like AQI, is primarily a sectarian organisation, dedicated to eradicating the Shia governments in Baghdad and the Alawite regime in Damascus.

Military analysts also point to the similarity in battleground tactics used by Isis with those used by AQI, in particular the way both deploy force in circles of pressure, particularly around cities, using waves of car and truck bombs.

For Fawaz Gerges, professor of international relations at the London School of Economics and an expert in al-Qaeda and Islamic extremism, the defining characteristic of the Isis/AQI approach, however is the their particular “use of violence”.

“Groups like al-Qaeda used violence in a tactical way, in a way proportional to their aims,” he says. “For Isis and AQI the savagery is the point. The action is what matters, not the ideas. To Zarqawi and Baghdadi [the Isis leader], the spectacle and the limitless force – beheadings, crucifications, people being buried alive – is what matters.”