Nairobi shopping mall attack: a new source of terror What the wider world was witnessing, meanwhile, was the coming to age of a group that was all but unknown 10 years ago, but whose sympathisers in Somalia's vast diasporas - including Britain - now pose as much of a threat to Western interests as any other al-Qaeda franchise. Nor is al-Shabaab's influence restricted purely to Somalis - two years ago, the same Kenyan police force that is now besieging the Westlands shopping mall arrested a young Nigerian-born Briton for attempting to cross the Somalia border to join the group. Scotland Yard fears that up to 100 other Britons may have been trained in Somalia with al-Shabaab, raising the prospect of a more organised terror strike in the future. "If we don't act now, there could be devastating results," warned Dr Razaq Raj, an expert on Islamic terrorism at Leeds Metropolitan University. The international community must think about how to challenge al-Shabaab. So who exactly are Harakat al-Shabaab al-Mujahideen, to give them their full title? The shortened version of their name translates as "The Youth" - the current generation of which in Somalia has grown up with some of the bleakest prospects on the planet. Most have known nothing but anarchy in their home country, which has been without a properly functioning government since 1991. It is perhaps no surprise, therefore, that many have broken with Somalia's long-standing tradition of moderate Sufi Islam to embrace the more militant strains exported from the Saudi peninsula, which has set up vast numbers of madrassas in Somalia throughout the past 20 years. I first heard of al-Shabaab on a trip to Mogadishu in 2006, when a coalition of Islamists - including moderates - had managed to impose peace after nearly a decade-and-a-half of warlord thuggery. As we saw during a visit to a barracks outside the capital, the Islamists" trick was partly to put warlords" ex-footsoldiers through religious "bootcamp," converting yesterday's murderers, robbers and rapists into tomorrow's holy warriors. But alongside the fragile peace came Taliban-style strictures banning music, dancing and most other kinds of fun. Fearing that Somalia's new Islamist overlords would also turn it into a haven for al-Qaeda, in early 2007 Washington authorised an invasion by neighbouring Ethiopia. Islamist rule was replaced with a transitional government propped up by United Nations mandate, Western cash, and African Union troops from Somalia's mainly Christian neighbours. The invasion also had the effect, though, of turning al-Shabaab into an all-out guerrilla movement, divorced from its more moderate allies. It began a vicious insurgency against the transitional government, and also seized control of much of Mogadishu and swathes of southern Somalia, dreaming up edicts as ludicrous as anything imposed by the Taliban. Teenage girls would be stoned to death for adultery, women were banned from wearing bras on the basis that they showcased the chest, and in 2010, men were even forbidden from watching the South Africa World Cup. In the stern words of al-Shabaab spokesman, "They will not benefit anything or get any experience by watching mad men jumping up and down." For children, meanwhile, one of the few acceptable forms of entertainment was Koranic recital contests, for which prizes would include guns, grenades and land mines to use against the "infidel" African Union forces. With the piety also came hypocrisy. While publicly condemning the piracy industry that boomed in Somalia from 2008, al-Shabaab is also thought to have quietly taken fat slices of ransom payments in exchange for turning a blind eye to buccaneers on their turf. In the last two years, the movement has been somewhat on the back foot, losing control of Mogadishu and alienating even its own followers through its refusal to let foreign aid agencies operate in the country, a policy that caused a widespread famine. But while its domestic fortunes have waned, its international agenda has grown in tandem with other African militant groups, such as Nigeria's Boko Haram and Mali's al-Qaeda in the Islamic Maghreb. During the 2010 World Cup, al-Shabaab bombers carried out their first major attack abroad, killing 74 people in an attack in Uganda, which contributed troops to Somalia's African Union force. As Ahmed Abdi Godane, an al-Shabaab leader implicated in the murder of two British aid workers 10 years ago, put it: "What happened in Kampala is just the beginning." Events over the weekend have now proved him right. The fear now, though, is how it might end. Already there are fears of reprisals against Kenya's Somalis, hundreds of thousands of whom have fled there in recent years precisely to escape al-Shabaab. Sustained terror campaigns there could also devastate the country's vital tourism sector, destabilising an economy already reeling from the world financial crisis. Closer to home, though, there will be fears that the success of such a terrorist "spectacular" could act as a recruiting sergeant for impressionable young Somalis in Britain. From London's Wood Green through to Cardiff's Grangetown and Manchester's Cheetham Hill, an estimated 100,000 Somalis are now resident in Britain - many of them jobless, and suffering the same disaffection that drove the Pakistani contingent of the July 7 bombers. Somali youth leaders have warned me in the past that they fear such an attack is only a question of time - warnings that acquire an added credence when one follows the sneers and insults on al-Shabaab's English-language Twitter feed. Not only are many of its postings surprisingly literate for a jihadi group based in the Somali bush, they bear numerous jokes, slang and linguistic tics that are distinctly British. Someone, somewhere in Somalia's British diaspora, apparently finds al-Shabaab's Nairobi atrocities, however sickening to us, very funny indeed.