A number of recent terrorist attacks were committed by young men and women who had radicalized, went to train with IS in the Middle East, then returned to their home country to commit acts of violence. In this text, Phil Gurski examines why some people decide to abandon their homeland to join terrorist groups, and whether they pose a significant threat to their societies if they survive and return. The focus is on Canadians and other Westerners who see violent Jihad as divine obligation, with the intention to challenge the view that foreign fighters are all brainwashed. The book first looks at state motivation for resorting to conflict and the nature of war, including Jihad. It then discusses why Westerners volunteered to join the military in past wars to offer points of comparison before focusing on why some are now going to Iraq and Syria to fight along groups such as Islamic State. This includes a thorough discussion of the increasing participation of women and the debates among extremists on whether they can engage in warfare. Lastly, the threat posed by radicalized fighters when they return home after either training or waging war abroad is examined in detail along with what is done to prevent and counter it. Written in an accessible manner by a reputed expert on terrorism and radicalization, the text will appeal to anyone seeking to understand why people join terrorist groups and the threats they represent to their homeland.