A salient feature of armed conflict in the Muslim world since 1980 is the involvement of so-called foreign fighters, that is, unpaid combatants with no apparent link to the conflict other than religious affinity with the Muslim side. Since 1980 between 10,000 and 30,000 such fighters have inserted themselves into conflicts from Bosnia in the west to the Philippines in the east. Foreign fighters matter because they can affect the conflicts they join, as they did in post-2003 Iraq by promoting sectarian violence and indiscriminate tactics. 1.Perhaps more important, foreign fighter mobilizations empower transnational terrorist groups such as al-Qaida, because volunteering for war is the principal stepping-stone for individual involvement in more extreme forms of militancy. For example, when Muslims in the West radicalize, they usually do not plot attacks in their home country right away, but travel to a war zone such as Iraq or Afghanistan first. Indeed, a majority of al-Qaida operatives began their militant careers as war volunteers, and most transnational jihadi groups today are by-products of foreign fighter mobilizations. 2.Foreign fighters are therefore key to understanding transnational Islamist militancy.