Thucydides (c. 460 B.C. – c. 395 B.C.) (Greek Θουκυδίδης, Thoukydídēs) was a Greek historian and author of the History of the Peloponnesian War, which recounts the 5th century B.C. war between Sparta and Athens to the year 411 B.C. Thucydides has been dubbed the father of "scientific history" due to 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 classical 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, genocide (as practised against the Melians), and civil war.

Written four hundred years before the birth of Christ, this detailed contemporary account of the struggle between Athens and Sparta stands an excellent chance of fulfilling the author's ambitious claim that the work "was done to last forever." The conflicts between the two empires over shipping, trade, and colonial expansion came to a head in 431 b.c. in Northern Greece, and the entire Greek world was plunged into 27 years of war. Thucydides applied a passion for accuracy and a contempt for myth and romance in compiling this exhaustively factual record of the disastrous conflict that eventually ended the Athenian empire.