This thesis provides a comparative Foreign Policy Analysis (FPA) framework to explain the reasons behind the US and the UK decisions to invade Iraq in 2003. It suggests that groupthink, leadership style, and analogical reasoning account for the degrees in convergence and divergences in the decision-making process in both countries. While international factors contributed to the decision, to truly understand how these two governments made it, it is necessary to examine the individuals who made the decision. This research uses a structured, focused comparison process tracing data analysis method to scrutinise this case, enabling a more comprehensive explanation of the case. As such, this research contributes on a theoretical and empirical level. Theoretically, this research responds to the challenges laid down for FPA: to establish links between theoretical models, to compare national contexts, and to speak to policy practitioners. To make this contribution, this thesis shows the usefulness of the comparative FPA framework used to explain the parallels and deviations in decision-making processes in each country. Empirically, this research contributes a rich analysis of both decision-making processes that led to the invasion of Iraq in 2003, using recently declassified documentary materials and interviews with decision-makers involved.