In the discipline of International Relations (IR), realism is a school of thought that emphasises the competitive and conflictual side of international relations. Realism’s roots are often said to be found in some of humankind’s earliest historical writings, particularly Thucydides’ history of the Peloponnesian War, which raged between 431 and 404 BCE. Thucydides, writing over two thousand years ago, was not a ‘realist’ because IR theory did not exist in named form until the twentieth century. However, when looking back from a contemporary vantage point, theorists detected many similarities in the thought patterns and behaviours of the ancient world and the modern world. They then drew on his writings, and that of others, to lend weight to the idea that there was a timeless theory spanning all recorded human history. That theory was named ‘realism’.