The arrest of four men by the British authorities on suspicion of orchestrating a terrorist attack once again highlights the threat of Western extremists in Iraq and Syria, and the risk they pose to their respective homelands.
Since the start of the Syrian civil war in 2011, foreign militants from across the globe have travelled to Syria to fight the Assad regime.
According to a report authored by members of the 9/11 commission, the civil war in Syria has attracted around 10,000 foreign fighters from more than 80 countries. A growing number of these foreign fighter contingents have also returned to Iraq, determined to reignite sectarian tension in the region.
While the majority of foreign fighters are Arabs, the influx of Westerners and non-European Westerners (from Australia, Canada and the United States) is significant.
A study done by the International Centre for the Study of Radicalisation at King's College in London on the number of foreign fighters in Syria found that 10 to 15 per cent of the foreign fighters are from the West. Britons make up one of the biggest groups of Western fighters, with Danes, Italians and the French not far behind.
The news that American Douglas McCain was killed while fighting in Syria also indicates that there are Americans currently in Syria fighting against the Assad regime.
In February this year, Director of National Intelligence James Clapper told the US Congress that more than 50 Americans are thought to be fighting in Syria.
Australian Foreign Minister Julie Bishop estimates that 150 Australians have been or are currently overseas fighting with extremists in Iraq and Syria.
The influx of overseas extremists is unprecedented. The figures exceed the number of foreign militants involved in Afghanistan during its decade of war and its subsequent violent aftermath. Unlike in Afghanistan, many travelled overseas not to just train or provide financial or logistical support but to participate in the conflict directly.
There are many reasons why so many individuals have travelled to Iraq and Syria to fight.
Many have been drawn in by predictions in a version of Islamic ideology that the apocalypse will take place in Greater Syria. Such narrative has been inflamed by stories of atrocities against Sunni Muslims alleged to be committed by the Alawite Assad regime.
Accessibility is also a factor. In contrast to other theatres such as in Afghanistan, Mali and Somalia, Iraq and Syria are much more logistically accessible.
Europol reports that many foreign militants interested in joining the fray have travelled through Turkey, a common vacation destination, which arouses no or limited suspicion.
Most of the foreign extremists have been assimilated into ISIL, also referred to as ISIS (Islamic State in Iraq and Syria), but not exclusively. Some have joined other rebel groups such as the Jabhat al-Nusra (or al-Nusra Front) and Ahrar al-Sham.
These groups were founded by individuals who at one time were senior members of Al-Qaeda. They tend to be more inclusive, highly organised, and much better financed than their more moderate counterparts, such as the Free Syrian Army.