In the wake of the Taliban's military defeat in 2001, foreign fighters played a critical role in assisting the Taliban to launch an insurgency against Coalition forces. Ten years later, by al-Qaida's own admission, the Taliban "almost didn't need" al-Qaida's non-Afghan fighters. Over time the Taliban grew sufficiently in strength that its need for foreign fighters—and foreign fighters' influence—virtually disappeared. Somalia shows a similar pattern. Foreign fighters initially played a prominent role in al-Shabaab, helping the group to launch an insurgency against Ethiopian forces, but their influence also declined as al-Shabaab became the dominant insurgent organization and built ties within Somali society. This is the first book to examine how foreign fighters gain and lose influence during insurgencies. Understanding foreign fighters' impact on conflicts is of increasing importance as the number of foreign fighters who have mobilized has grown in recent years, both in absolute numbers and in terms of the proportion of conflicts in which they are involved. In examining the conditions that contribute to the changes in their effect over time, Bacon explains how and why foreign fighter influence evolves within a conflict and which factors enable and constrain foreign fighter influence throughout an insurgency. Knowing how foreign fighters are situated vis-à-vis local insurgents, specifically the type of relationships they forge, should shape every aspect of counterinsurgency strategies to avoid counterproductive tactics, more effectively counter insurgent movements, and better protect civilians.