This study provides a comparative study of the foreign policy of the United States and the European Union in Iraq (2003–2023). After the 2003 invasion, Iraq became the main focus of competition between global and regional powers. American foreign policy, based on the model of realism and a focus on hard power (military interventions and political pressure), emphasized ensuring security and creating a new political order. In contrast, the European Union, with its approach of liberalism and the use of soft power (diplomatic cooperation and human development), focused on strengthening democratic institutions and social reconstruction in the country. This difference in approaches, which can clearly be reconciled with the theoretical opposition between realism and liberalism, has led to different results. This study also analyzes the role of regional actors (Iran, Turkey, Saudi Arabia) and their impact on US and EU policies, and shows how foreign interventions, along with the internal complexities of Iraq, have led to the formation of a multipolar and complex political and security situation. Finally, this research attempts to show how the differences in the approaches of the United States (security-oriented) and the European Union (development-oriented) have had different impacts on the stability, security, and development of Iraq over the last two decades.