This study follows the human side of ISIS’s amazing rise and fall in Iraq. It does not do this as a battle-by-battle history of the group, but rather explores how common Iraqis viewed the group, interacted with the group, joined the group, fought in the group, and ultimately suffered under its brutal rule. Specifically, we seek to answer the following questions: What factors conditioned public support and opposition toward ISIS in Iraq? Why did some Iraqis move from passive support for the group to actively participating in the group’s activities, including fighting? This study uses a social psychological approach to understand the trajectory of ISIS in Iraq. The book argues that ISIS derived support from how it was perceived to either meet or threaten basic human needs for individual Iraqis. The three basic human needs in question are the physical needs for security and sustenance and the psychological need for a feeling of individual significance. Our analysis is based on a unique array of public opinion data from surveys, focus groups, and in-depth face-to-face interviews with forty detained ISIS foot soldiers and four senior leaders to explain why some Iraqis came to join and acquiesce to ISIS while others opposed it, why ISIS lost the hearts and minds of Iraqi Sunni Arabs, and how this contributed decidedly to its battlefield defeats.