2. Returned fighters could take matters into their own hands

The most immediate threat to the West comes from hundreds of extremists who have returned home after fighting with terrorist groups in Syria. While little evidence has emerged so far that ISIS has directed them to launch attacks, their urban warfare skills would make them especially dangerous.

The first terrorist attack on Western soil linked to Syria probably followed this trajectory. Mehdi Nemmouche, a French-Algerian who spent a year in Syria and was recruited into ISIS, has been charged with gunning down four people at a Jewish museum in Brussels, Belgium, in May. ISIS itself did not claim responsibility for the attack, suggesting to investigators that Nemmouche planned the attack himself.

Among Western countries Europe has by far the greatest number of returnees.