This chapter examines the central assumptions of classical realism by analysing the texts of ancient and modern writers and contrasting their ideas with neorealism and other variants of modern realism. Classical realism represents an approach to international relations that dates back to Thucydides and his account of the Peloponnesian War. According to classical realists, power plays a major role in politics, but they also acknowledge its limitations and the ways it can be self-defeating. The chapter begins with a discussion of the position of classical realists regarding order and stability, focusing on the views of Thucydides and Hans J. Morgenthau with respect to the concepts of community, balance of power, and interest and justice. It then considers what classical realists think about change and transformation as well as the nature and purpose of theory. It concludes by commenting on the Iraq war in the context of classical realism.