Within hours of Wednesday’s attack in Westminster, social media channels used by Islamic extremist activists and sympathisers were flooded with new images of London “under attack”. Quickly produced with design software, they showed the Houses of Parliament and other landmarks shattered by explosions and wreathed in smoke.

The UK has always remained near the top of the list of priority targets for internationally focused Islamic militant groups. That there would be celebration of a successful attack on such a symbolic target in the heart of the British capital was inevitable.

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But the jubilation masked something important: the images showed the type of attack extremists wanted to happen, not what actually occurred.

Britain has not suffered any major terrorist attacks since the 2005 bombings on London transport which killed 52, as well as four bombers. Until last week Britain’s only casualty from Islamic extremist terrorism on home soil in over a decade was Lee Rigby, an off-duty soldier run down by a car and then stabbed and hacked to death in south-east London in 2013.

This is due to various factors. One has been the the declining influence of al–Qaida, the veteran group responsible for the 9/11 strikes, which was based in Pakistan, where around a million Britons have roots. By 2011, al-Qaida was hugely weakened and the connection between Britain and Pakistan was less relevant. By 2014 the group had been supplanted as the world’s leading jihadi organisation by Isis, while Afghanistan and Pakistan had been replaced by Syria and Iraq as the crucible of global jihad.

Another factor has been the reduced role played by the UK in Afghanistan and Iraq – wars that at their height acted as a recruiting sergeant for jihadi groups.

New counter-terrorist strategies after the 2005 attacks, more resources and legislation also made a significant difference.

Despite their many differences over strategy, Isis and al-Qaida still share a desire to attack the UK. The problem for Isis, which appears to have begun contemplating a campaign of terrorist violence in the west even before seizing the Iraqi city of Mosul and declaring a caliphate almost 33 months ago, was to find the way to penetrate the reinforced defences of the UK.

Until this week, they were struggling to do this, and it was France which bore the brunt of their offensive.

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Why? As ever there are many reasons. France was more accessible, and its intelligence services in some disarray. But the crucial difference was human resources. A group of senior commanders within Isis were French, with a particular animus against their native land. Though there were hundreds of Britons who had travelled to Syria – around 850 since 2014 in total, authorities say today – there were almost twice as many French nationals and large numbers of Francophone Belgians too. The latter particularly were linked to an extensive network of supporters back home, and included several talented organisers. Weak border controls and poor intelligence-sharing between EU states meant they could travel easily to and from their homelands. Early Isis attempts to launch attacks failed. But its commanders learned fast, and more than 250 deaths in France and Belgium were the consequence.

That in 2015 a Belgian recruit may have been employed by Isis to survey possible targets in Britain – not a UK citizen – underlines how limited its resources were in the UK. Nor have any British recruits reached the high ranks in the group of their French counterparts.

Like al-Qaida before it, Isis is now much weakened by the onslaught from western and regional powers provoked by its international attacks. The French-Belgian network is, officials claim, dismantled.

But as the military tide began to turn against Isis, so its tactics have changed. The language of the group’s claim of responsibility does not suggest that the group had prior knowledge of last week’s attack in London. This is important. Back in 2014, Abu Muhammad al-Adnani, a senior Isis spokesman and strategist, called on sympathisers in the west to strike locally, rather than set out to migrate to the caliphate. He told them to attack “unbelievers” with whatever came to hand.

“If you are not able to find a bomb or a bullet, then smash his head with a rock, or slaughter him with a knife, or run him over with your car, or throw him down from a high place, or choke him, or poison him,” he said.

Facebook Twitter Pinterest An Islamic State police car destroyed after heavy airstrikes in Mosul, Iraq. Photograph: Osie Greenway / Barcroft Images

In mid-2016 Adnani repeated his call for individual “DIY” attacks. It was answered in Nice in July when a stolen truck was driven through a Bastille Day parade killing 86 people and in Berlin in December, where 12 died in a similar attack. The US too has seen a string of attacks by individuals who have declared their allegiance to Isis before launching strikes in which scores have died.

It is possible Khalid Masood, the 52-year-old who killed four in London last week, was responding directly to Adnani’s call, perhaps relayed through social media. It is equally likely he was inspired by the example of earlier attacks. The “copycat effect” in terrorism is powerful.

Such strikes still help Isis achieve its treble aim of terrorising enemies, mobilising supporters and polarising communities to foment sectarian hate. There is something uniquely terrifying about a familiar object – a car, for example – becoming a deadly threat, as directors of horror films have long known. The obvious difficulty of spotting or stopping the apparently “lone attacker” makes the threat seem ubiquitous and that much scarier. A series of smaller attacks can thus have the same, if not greater, effect than a single spectacular major strike.

But this cannot disguise the fact that the group is in retreat. Isis is losing territory and control over populations and key resources. It will endure in some more limited form for many years to come – and pose a threat too – but the celebratory images of the Houses of Parliament in flames reveal a failure and a defeat – not a victory.