Intrastate conflicts, such as civil wars and ethnic confrontations, are the predominant form of organized violence in the world today. But internal strife can destabilize entire regions, drawing in people living beyond state borders--particularly those who share ideology, ethnicity, or kinship with one of the groups involved. These nonstate actors may not be enlisted in formal armies or political parties, but they can play a significant role in a conflict. For example, when foreign volunteers forge alliances with domestic groups, they tend to attract other foreign interventions and may...