In conflict zones around the world, thousands of insurgents have been arriving from outside to fight on behalf of local rebel groups. They have been an increasing source of concern because they engage in deadlier attacks than local fighters do, violate international laws and norms of citizenship, and are not deterred by the advantages of their adversaries, the strongest countries in the world. Foreign fighters of this description have made headlines in recent wars in Afghanistan, Iraq, and Somalia, and the term is widely equated with militant Islamists. However, foreign fighters are not a new phenomenon, and nearly identical types of identical insurgents have also been fighting throughout modern history for other causes such as communism and ethnic groups. Examining the long history of foreign fighters provides answers about how they join insurgencies, why they behave in the way that they do, and what policymakers can do in response. This first comprehensive study of foreign fighters examines how insurgencies recruit individuals who would seem to have no direct connection to their distant wars. It examines why the same recruiting strategies are used successfully in all cases of foreign fighters, no matter what the circumstances are of the conflict in which they appear. It catalogues foreign fighters in civil wars over the past two centuries, providing data indicating that they are disproportionately successful and growing in number. Detailed case histories using archival material and original interviews demonstrates the same recruitment patterns in highly diverse conflicts including the Texas Revolution, the Spanish Civil War, the Israeli War of Independence, and the Afghanistan War. The results show that foreign fighters from Davy Crockett to George Orwell to Osama bin Laden create and respond to strategically crafted appeals to defend transnational communities under dire threat.