This paper examines the problem of causation in IR as an epistemological problem. It seeks to engage complexity science/theory within the current debate. It draws attention to the key lessons that can be learned in order to rethink the traditional Humean conception of causation, which has dominated most of the mainstream theories. The paper is divided into three main sections. The first section offers an introduction to causation as an epistemological problem in IR. The second section seeks to introduce an alternative concept of causation, based on the contributions of critical realism, and draws some lessons from complexity theory. The third section, as a conclusion, seeks to review the causes of the end of the Cold War as an essential part of the dominant debate on causation problematic in IR.