This book aims to reinvigorate realist international relations theory by developing a catalogue of micro-mechanisms able to explain security policy decision-making.Typically, realism discounts the role of individuals and uses states as the unit of analysis. By examining instead the mental operations of those who act on behalf of the state, a better understanding of security policy formation is attainable. The book demonstrates how realism can be translated from a systemic "grand theory" into a catalogue of psychologically plausible mechanisms applicable to individual decision-makers. This catalogue, here called "Mechanistic Realism", may be employed to investigate the cognitive precursors to security policy.The explanatory power of Mechanistic Realism is demonstrated through a meticulous analysis of what transpired inside the George W. Bush administration, as its members forged a response to the 2001 terrorist attacks. Through the exploration of individual-level data, Mechanistic Realism provides a more comprehensive analysis of the US response. The book concludes that international relations (IR) scholars would benefit analytically by assembling the most pertinent mechanisms into an explanatory toolbox rather than developing and applying grand theories. Mechanistic Realism is a first step in this direction.This book should be of great interest to students of IR, foreign policy, American politics, and security studies in general. .