brilliant but unscrupulous Athenian politician and military commander who provoked the sharp political antagonisms at Athens that were the main causes of Athens' defeat by Sparta in the Peloponnesian War (431–404 BC).

Well-born and wealthy, Alcibiades was only a small boy when his father—who was in command of the Athenian army—was killed in 447 or 446 BC, at Coronea, Boeotia. Alcibiades' guardian, the statesman Pericles, a distant relation, was too preoccupied with political leadership to provide the guidance and affection that the boy needed. As he grew up, Alcibiades was strikingly handsome and keen witted, but he was extravagant, irresponsible, and self-centred as well. He was, however, impressed by the moral strength and the keen mind of the philosopher Socrates, who, in turn, was strongly attracted by Alcibiades' beauty and intellectual promise. They served together at Potidaea (432) in the Chalcidice region, where Alcibiades was defended by Socrates when he was wounded, a debt that he repaid when he stayed to protect Socrates in the flight from the Battle of Delium (424), north of Athens. Yet before he was 30 he had abandoned the intellectual integrity that Socrates demanded, in favour of the rewards of the kind of politics that Socrates despised.

During the 420s Alcibiades was best known for his personal extravagance and his courage in battle; but he had also become a recognized speaker in the Ecclesia (assembly), and as Athens moved toward peace, he hoped that the ties that had once existed between his family and Sparta would enable him to secure the credit for bringing peace to Athens. According to the historian Thucydides, who knew Alcibiades well and judged him dispassionately, it was the fact that the Spartans instead chose to negotiate through established political leaders that dictated Alcibiades' subsequent choice of policies.

General for the first time in 420, he opposed the aristocratic leader Nicias, who had negotiated peace, and steered Athens into an anti-Spartan alliance with Argos, Elis, and Mantineia, three city-states of the Peloponnese. This alliance was defeated by Sparta at the Battle of Mantineia (418). Alcibiades, however, escaped ostracism, a form of banishment, by joining forces with Nicias against Hyperbolus, the successor of the demagogue politician Cleon as champion of the common people. In 416 Alcibiades restored his reputation by entering seven chariots at Olympia and taking first, second, and fourth places. This made it easier for him, in 415, to persuade the Athenians to send a major military expedition to Sicily against the city of Syracuse. He was appointed to share the command, but, shortly before the expedition was due to sail, the hermae (herm) (busts of Hermes, messenger of Zeus and patron of all who use the roads, set up in public places throughout the city) were found to have been mutilated. In the ensuing panic Alcibiades was accused of being the originator of the sacrilege as well as of having profaned the Eleusinian Mysteries. He demanded an immediate inquiry, but his enemies, led by Androcles (the successor of Hyperbolus), ensured that he sailed with the charge still hanging over him. Shortly after reaching Sicily, he was recalled; but on the journey home he escaped and, learning that he had been condemned in absentia to death, went to Sparta. There he advised the Spartans to send a general to help the Syracusans and also to fortify Decelea in Attica, two serious blows to Athens. He also confirmed his reputation with women (which the rich Athenian whom he had married appreciated only too well) by seducing the wife of the Spartan king Agis II, who was at Decelea with his army.

In 412 Alcibiades helped stir up revolt among Athenian allies in Ionia, on the west coast of Asia Minor; but Sparta now turned against him, and he moved to Sardis to exercise his charm on the Persian governor. When some Athenian officers in the fleet began to plan an oligarchic coup, he held out hopes that if the democracy was overthrown he could secure financial support from Persia. In this he failed and, discarded by the oligarchs who had seized power, he was recalled by the Athenian fleet, which remained loyal to the democracy and needed his abilities. From 411 to 408 he helped Athens to a spectacular recovery, defeating the Spartan fleet in the Hellespont at Abydos (411) and Cyzicus (410) and regaining control over the vital grain route from the Black Sea. These successes encouraged him to return in 407 to Athens, where he was welcomed with enthusiasm and given supreme control of the conduct of the war. In a typically bold gesture he led the procession to the Eleusinian festival by road in spite of the danger from the Spartan force at Decelea; but, in the same year, after a minor naval defeat in his absence, his political enemies persuaded the people to reject him, and he retired to a castle in Thrace. He remained, however, a disturbing influence on Athenian politics and destroyed any hopes of a political consensus. When the Athenians at Aegospotami (Aegospotami, Battle of) (405) facing the Spartans in the Hellespont grew increasingly careless, he warned them of their danger. But he was ignored, and, when the Athenians lost their whole fleet in a surprise attack by the Spartan admiral Lysander, Alcibiades was no longer safe in his Thracian castle. He took refuge in Phrygia in northwestern Asia Minor with the Persian governor, who was induced by the Spartans to have him murdered.

Perhaps the most gifted Athenian of his generation, Alcibiades possessed great charm and brilliant political and military abilities but was absolutely unscrupulous. His advice, whether to Athens or Sparta, oligarchs or democrats, was dictated by selfish motives, and the Athenians could never trust him enough to take advantage of his talents. Moreover, the radical leader Cleon and his successors carried on a bitter feud with him, which at the critical period undermined Athenian confidence. Alcibiades could not practice his master's virtues, and his example of undisciplined and restless ambition strengthened the charge brought against Socrates in 399 of corrupting the youth of Athens.