Few works can claim to form the foundation stones of one entire academic discipline, let alone two, but Thucydides’s celebrated History of the Peloponnesian War is not only one of the first great works of history, but also the departure point from which the modern discipline of international relations has been built. This is the case largely because the author is a master of analysis; setting out with the aim of giving a clear, well-reasoned account of one of the seminal events of the age – a war that resulted in the collapse of Athenian power and the rise of Sparta – Thucydides took care to build a single, beautifully-structured argument that was faithful to chronology and took remarkably few liberties with the source materials. He avoided the sort of assumptions that make earlier works frustrating for modern scholars, for example seeking reasons for outcomes that were rooted in human actions and agency, not in the will of the gods. And he was careful to explain where he had obtained much of his information. As a work of structure – and as a work of reasoning – The History of the Peloponnesian War continues to inspire, be read and be taught more than 2, 000 years after it was written.