Friday marks the anniversary of the assassination of Osama bin Laden in Pakistan by U.S. forces . Three years on , the core organization of al Qaeda has been significantly depleted . However , the danger from so-called `` home-grown '' terrorism may yet be on the rise , fuelled by foreign nationals returning from key international theatres of war such as Syria . In the United Kingdom , for instance , Charles Farr , director of the Office for Security and Counter-terrorism , recently asserted that the threat from UK nationals travelling to participate in the Syrian conflict is the `` biggest challenge '' to UK security services since 9/11 . The total number of UK nationals who have fought in Syria is estimated at up to 366 by the International Centre for the Study of Radicalisation ( ICSR ) at Kings College London , with a significant number believed to have been killed there . Moreover , ICSR estimates that as many as 11,000 foreign fighters overall may have fought in Syria , from more than 70 countries , a number which is reported to be a higher concentration than anywhere since Afghanistan in the 1980s . A central concern here is that many of these individuals , which include potentially as many as 2,000 from Western Europe , plus individuals from North America , Australia , South-east Asia , and Africa , will return from Syria to their respective homelands battle hardened with significantly greater terrorist capability and resolve . And given that the relatively large number of these foreign fighters makes them collectively difficult to track with precision , further home-grown attacks appear increasingly likely . While this offers propaganda value for al Qaeda , such individuals often lack thorough indoctrination of the network 's core messages , even though some , including last year 's London and Boston terrorists , appear to have been partially motivated or inspired by them . This reflects , in part , the continued diminution of al Qaeda 's central organization which is unlikely to be able to provide operational support for home-grown terrorists . It also means that these people , while highly dangerous , do not individually represent the same level of threat to international interests as larger cells and terrorist groups , and are thus less likely to be able to perform major , spectacular attacks in the mould of 9/11 . Thus , while bin Laden 's successor , Ayman al-Zawahiri , has sought to seize on home-grown Western terrorism , and indeed the chaos in Syria , for propaganda purposes , this can not disguise the central al Qaeda organization 's declining fortunes . Al-Zawahiri lacks bin Laden 's personal authority within the terrorist network , and the core has also been weakened by the assassination of numerous other senior terrorist leaders . A fundamental challenge for al-Zawahiri is that while the central al Qaeda leadership appears to remain located largely in Pakistani tribal areas and borderlands , the wider network has becoming increasingly de-centralized and dispersed . The problems this can cause were underlined earlier this month in Syria when a jihadist group called Islamic State of Iraq and Greater Syria ( ISIS ) accused al Qaeda of having `` deviated from the correct path , '' and `` divided the mujahedeen in every place . '' The origin of the dispute probably lies in an edict from al-Zawahiri to ISIS to confine its activities to Iraq after it was accused of abuses of civilians and rival rebels in Syria . Al-Zawahiri has instead recognized the al-Nusra Front as the official al Qaeda affiliate in Syria and called for jihadist unity behind it . Accompanying this dispersal and de-centralization has been shifting the focus of al Qaeda groups and franchises whose attention is more on `` local '' national or regional issues , rather than the broader international designs of bin Laden . In part , this also reflects the greater difficulty of attacking key international targets many of whose defences have significantly hardened since 9/11 . There has also been evolution in the geographical focal points of al Qaeda activity with terrorist nodes of growing importance , for instance , in key African and Middle Eastern countries , such as Yemen , where political upheaval since bin Laden 's death has allowed terrorists and other insurgents to secure greater foothold . And reflecting this changed risk pattern , U.S. forces are re-deploying as a result . For instance , the CIA has expanded its staff in Yemen , and also enhanced its air bases in the Gulf from which it can launch drone strikes into the country . Only last week , Washington launched a major drone attack which , according to the Yemeni government , killed at least two dozen militants , including foreign fighters , in an al Qaeda training camp in the remote mountainous area of Abyan . Meanwhile in Africa , U.S. forces have also scaled up facilities in numerous states , including Kenya , Ethiopia , and the Central African Republic . This is intended to allow for greater aerial surveillance coverage and drone strikes , especially in North Africa , plus sites for military hardware storage . So while the central core of al Qaeda has been diminished since bin Laden 's demise , much of the wider terrorist network remains potent , albeit more focused on local grievances than grander international ambitions . However , danger may be growing from home-grown threats fueled by battle-hardened individuals returning from foreign theaters of war , especially Syria , with greater terrorist resolve and capabilities .
Friday marks third anniversary of assassination of Osama bin Laden in Pakistan by U.S. forces . Al Qaeda is depleted , but danger of home-grown terror on rise , writes Andrew Hammond . TL;DR NEW: Al Qaeda franchises now focus more on `` local '' issues , Hammond says , threatening Western security .