Thomas Hegghammer of the Norwegian Defence Research Establishment recently found that most terrorists originating in the West (Europe, Australia, or the US) conduct their terrorism in conflict zones such as Iraq or Afghanistan. These terrorists are defined as “foreign fighters”. When these foreign fighter veterans return to the West, they are more likely to complete attacks, which are more likely to be lethal (American Political Science Review, volume 107, no. 1, Feb 2013, “Should I stay or should I go? Explaining variation in Western Jihadists’ choice between domestic and foreign fighting.”)
As the wars in Iraq and Afghanistan wind down, we can expect countervailing effects on terrorism in the West. On the one hand, there presumably will be less reason to conduct terrorism, as terrorists use these wars as justification for their actions. On the other hand, foreign fighter veterans will be returning to the West, increasing the quantity, militancy, and experience of the pool of potential domestic terrorists. New justifications for terrorism — for example Western intervention in Mali and Syria — can always be found by those so inclined.