Thucydides (; Ancient Greek: Θουκυδίδης, romanized: Thoukudídēs; c. 460 – c. 400 BC) was an Athenian historian and general. His History of the Peloponnesian War recounts the fifth-century BC war between Sparta and Athens until the year 411 BC. Thucydides has been dubbed the father of "scientific history" by those who accept his claims to have applied strict standards of impartiality and evidence-gathering and analysis of cause and effect, without reference to intervention by the gods, as outlined in his introduction to his work.

He also has been called the father of the school of political realism, which views the political behavior of individuals and the subsequent outcomes of relations between states as ultimately mediated by, and constructed upon, fear and self-interest. His text is still studied at universities and military colleges worldwide. The Melian dialogue is regarded as a seminal work of international relations theory, while his version of Pericles' Funeral Oration is widely studied by political theorists, historians, and students of the classics.

More generally, Thucydides developed an understanding of human nature to explain behavior in such crises as plagues, massacres, and civil war.