Thucydides (Greek Θουκυδίδης, Thoukudídēs) was a Greek historian and author from Alimos. 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 in terms 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 relations between nations as based on might rather than right. His text is still studied at advanced military colleges worldwide, and the Melian dialogue remains a seminal work of international relations theory.

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.