Every conflict has its signature tactic or weapon. The first world war had the trenches, the second the tank. In Vietnam, it was the helicopter. For the chaotic, interlinked conflicts of the decade that followed the 9/11 attacks, it was the suicide attack, whether in New York and Washington on that terrible day in 2001, on the London Underground, in a nightclub in Bali or, with much more murderous cumulative effect, again and again in Iraq, Afghanistan and Pakistan.

The signature tactic of this new phase of Islamic militancy and the war against it is still unclear at this early stage, but there is one obvious candidate: the ‘execution-style’ murder.

Such acts did occur, repeatedly, in the previous phase. The man thought to be responsible for planning the 9/11 attacks, Khalid Sheikh Mohammed, decapitated a US journalist, Daniel Pearl, only months after the strike in the US. The ancestors of the Islamic State, al-Qaida in Iraq, killed hostages, too. There were many other such killings that went unpublicised. The two Talibans – Afghan and Pakistani – have killed hundreds of so-called spies. So have other militants. The civil war in Iraq saw thousands summarily shot, burned and hanged.

But it was the suicide attacks, in the west and in the Islamic world, that marked the public imagination and came to symbolise the violence of the extremists. This was the key element introduced and popularised by Osama bin Laden and al-Qaida. The first suicide bombing in Afghanistan was in 2002. In 2010, there were 106.

This phase of militancy ended in 2011 with the outbreak of the Arab spring, the death of Bin Laden and the relative decline of his group, even if the broad ideology it had pioneered, rather than the tactics, continued to thrive.

But the primary chosen tool of the Islamic State to achieve this appears to be the execution murder. This is logical. The great difference between this phase of militancy and the last one is that most of the groups operating around the world today – from Boko Haram in Nigeria to al-Qaida in the Arabian Peninsula, from the Pakistani Taliban to Isis – prioritise seizing territory rather than opting for a strategy of “propaganda of the deed” designed to instigate violence and thus radicalise tens of millions of Muslims. Suicide bombings are used tactically for local aims, such as facilitating the seizure of a town or to undermine the morale of government troops, rather than strategically against distant western ones.

Rulers have long used public execution as a means of social control and to send a powerful message – to terrorise, mobilise and polarise. Think of the Romans, European rulers in the middle ages, the Tudors and the Stuarts, the Mughals and the Ottomans, Nazis, Communists, the Maoist regime, Iranian revolutionaries, the Taliban. The militants want to establish states and, more important, perhaps, be taken seriously as rulers of states.

In every theatre of jihadi conflict, there have been public killings, often filmed and broadcast. The Afghan Taliban has released videos of killings of women accused of adultery. A Libyan group was filmed shooting an alleged murderer last year. In Somalia in January, alleged spies were shot dead by the al-Shabaab group by a firing squad in a square in the town of Bardhere. Isis, of course, has killed hundreds of supposed “criminals”, including alleged adulterers and homosexuals as well as thieves and bandits. It has also killed thousands of Syrian, Iraqi and Kurdish captives.

Even the terrorist attacks that do occur in the west increasingly resemble public executions. In 2013, two men killed Lee Rigby, an off-duty soldier, in south-east London with a cleaver and knives. They also reportedly attempted to behead him. The attack on the offices of Charlie Hebdo in Paris last month saw 11 unarmed people gunned down by men with automatic rifles. A twelfth was wounded on the pavement and then shot in the head.

In both cases, efforts were made to ensure that the violence would be seen by large numbers. The Rigby attack took place in a street in daylight and the killers made no attempt to flee. One delivered a speech into the mobile phone of a passerby. The Charlie Hebdo attackers brought GoPro video cameras to the scene, although they did not use them. However, an associate did film his own attack on a kosher supermarket, in which he shot dead four shoppers two days later.

So where does it end? A horrific logic of escalation drove progressively larger mass casualty attacks on symbolic targets in the 1990s until 9/11. In the next decade, such strikes proved counter-productive, sapping support for al-Qaida and its offshoots rather than mobilising millions of new sympathisers.

It is unlikely that horrific acts such as the public murder of Muadh al-Kasasbeh will do much to bolster the legitimacy or popularity of Isis anywhere. But it took a decade before al-Qaida was marginalised and it may take as long before the wave of energy Isis has sent coursing through the Islamic extremist movement is reversed. In the meantime, the killings will continue.