A growing army of Islamist fighters, thousands of whom carry European passports, is busy learning the terror trade in areas of Iraq now controlled by the Islamic State of Iraq and Syria. European-born fighters have been instrumental in the organization’s stunning military successes so far, but their greatest contribution to the jihadist cause may come when they return home.

The ISIS is laying the groundwork for the reintroduction of potentially thousands of trained jihadists anxious to attack European targets. Some already have been hit, while other plans have been foiled. European governments have implemented limited countermeasures, but that might not be enough to disrupt large-scale future attacks in Europe and possibly the United States.