Thucydides (c. 460 BC - c. 395 BC) (Greek Θουκυδίδης, Thoukudídēs) was an ancient Greek historian, and the author of the History of the Peloponnesian War, which recounts the 5th century BC war between Sparta and Athens to the year 411 BC. Thucydides has been regarded as the father of "scientific history" because of his strict standards of gathering evidence and his analysis in terms of cause and effect without reference to intervention by the gods. He has also been considered the father of the school of political realism, which views the relations between nations as based on might rather than right. His classic text is still studied at advanced military colleges worldwide. More generally, he shows an interest in developing an understanding of human nature to explain human behavior in such crises as plague and civil war. Other scholars lay greater emphasis on the History’s elaborate literary artistry and the powerful rhetoric of its speeches and insist that its author exploited non-"scientific" literary genres no less than newer, rationalistic modes of explanation.

Thucydides (Greek: Θουκυδίδης) was a prominent politician of ancient Athens and the leader for a number of years of the powerful conservative faction.

Thucydides, the son of Melesias, was born in the ancient deme of Alopecê (Αλωπεκή) of Athens. The exact year of his birth is unknown, but his family was noble and he was a relative of Cimon, the charismatic general and leader of the conservative party. After Cimon's death, he succeeded him in the leadership of the conservatives and decided to exert a vehement opposition against Pericles, who was leading Athens, at the time.

Donald Kagan has identified Thucydides' party with the views expressed by the writer known as "pseudo-Xenophon" or "the Old Oligarch" in his Constitution of the Athenians, which some scholars believe to have been authored by Thucydides himself. Kagan believes that Thucydides' ultimate goal, which he could not state openly as doing so would alienate the pro-democratic majority, was to roll back the constitutional changes made by Ephialtes, reinstituting the more aristocratic and conservative government that had prevailed in Cimon's day.

Thucydides' political strength reached its peak in the wake of the First Peloponnesian War and the reorganization of the Athenian empire in the early 440s BC. Thucydides developed a new and effective political tactic by having his supporters sit together in the assembly, increasing their apparent strength and giving them a united voice. Kagan asserts that this tactic helped Thucydides mount a concerted opposition to Pericles which brought to light ideological differences among Pericles' supporters.

In 444 BC, the conservative and the democratic parties confronted each other in a fierce battle. Though some modern scholars doubt the details of Plutarch's account, according to Plutarch, Thucydides, the new leader of the conservatives, accused Pericles, the leader of the democrats, of profligacy, criticizing the way Pericles spent the money for his ambitious building plan. Thucydides managed to incite the passions of the Athenian Assembly in his favor, but when Pericles took the floor, the atmosphere immediately changed. Pericles proposed to pay for all the construction from his own purse, under the term that all these monuments would belong to him and not to Athens. The public applauded his stance and Thucydides suffered an unexpected defeat from the charismatic orator . As a result of his failure in confronting Pericles, Thucydides was ostracized for ten years, in 442 BC, and Pericles once again stood unchallenged in the Athenian political arena. Plutarch relates that, when Thucydides was asked by Sparta's king, Archidamus II, if he or Pericles was a better fighter, Thucydides answered without any hesitation that Pericles was a better fighter, because, even when he is defeated, he achieves to convince the audience that he won!

After being ostracized, Thucydides is said to have travelled to Sybaris, a city of Magna Graecia on the Gulf of Taranto in Italy, or Aegina, but this is unconfirmed .

While in Athens, Thucydides is also said to have accused Pericles' personal friend, Anaxagoras, of atheism and sympathy for the Persians.