Realism is a perspective in International Relations that emphasizes the role of power, national interest, and self-help in shaping state behavior. Realists view the international system as anarchic, with no central authority to enforce rules or enforce cooperation. They believe that states are motivated primarily by the pursuit of their own national interests, and that they will use military force, economic power, and other means to protect and promote these interests. Realism has been a dominant perspective in international relations since the early 20th century and continues to shape policy and academic discussions about world politics.
Source: Morgenthau, H. J. (1948). Politics among nations: the struggle for power and peace. New York: Alfred A. Knopf.