This book aims at understanding foreign fighters' motivations to mobilize and assessing their impact on internal armed conflict and in the coalition warfare during World War II. The Spanish Civil War is the most thoroughly analyzed case study. David Malet claims that transnational combatants who joined the Republican Army against Francisco Franco in the Spanish Civil War did not have a direct impact on the battlefield but prolonged the war and discouraged Franco from intervening alongside the Axis. Nir Arielli argues that foreign fighters were one of the most important features of the Spanish Civil War for media and public opinion. Samuël Kruizinga notes that the Dutch foreign fighters in the Spanish Civil War were ideologically committed – despite not being directly associated with communism – and poorly trained and that they felt a sense of alienation due to language and cultural differences. Other contributions focus on foreign fighters' motivations in the 1899–1902 South African conflict, the 1861–64 American Civil War and in the Ottoman Army. Finally, the last section analyzes the foreign presence in the coalition warfare during World War II. The book offers a broad political and historical analysis and also contributes to security studies. Further contributions in this field might consider whether and how the military impact has changed over time, post-war trajectories, and how foreign fighters are considered in the public memory in both the state at war and their home countries.