Thucydides (/θjuːˈsɪdᵻdiːz/; Greek: Θουκυδίδης, Thoukudídēs, Ancient Greek: [tʰuːkydídɛːs]; c. 460 – c. 400 BC) was an Athenian historian and general. His History of the Peloponnesian War recounts the 5th century BC war between Sparta and Athens to the year 411 BC. Thucydides has been dubbed the father of "scientific history" because of his strict standards of evidence-gathering and analysis of cause and effect without reference to intervention by the gods, as outlined in his introduction to his work.

He has also been called the father of the school of political realism, which views the political behavior of individuals and the subsequent outcome of relations between states as ultimately mediated by and constructed upon the emotions of fear and self-interest.[3] His text is still studied at both universities and advanced military colleges worldwide. The Melian dialogue remains a seminal work of international relations theory while Pericles' Funeral Oration is widely studied in political theory, history, and classical studies.

More generally, Thucydides showed an interest in developing an understanding of human nature to explain behaviour in such crises as plague, massacres, as in that of the Melians, and civil war.