More than 20 years have passed since the 9/11 attacks, and people from all walks of life have conducted extensive discussions and research on the event. These discussions involve multiple fields, especially analyzing this historical event within the framework of international relations theory. Therefore, this paper will analyze the three major international relations theories of realism, liberalism, and constructivism, and determine which theory is most suitable for explaining the background, impact, and subsequent development of the 9/11 attacks. Specifically, this paper will argue that realism is the most appropriate theory as it emphasizes the role of national security and power, and can explain the tough military response taken by the United States after the 9/11 attacks. Meanwhile, this paper will also briefly explain why liberalism and constructivism are not suitable for analyzing the 9/11 attacks. Through this comparative analysis, this paper aims to reveal the unique advantages of realism in understanding and interpreting the 9/11 attacks.