Alcibiades, the son of Cleinias, was not yet twenty years old when the Archidamian War (the first phase of the Peloponnesian War) broke out. With his mentor Socrates, he was present when general Phormio besieged Potideia. Later, the two were present during the Battle of Delium, where the Athenians were defeated by the Thebans.
After the death of Alcibiades' uncle Pericles (429), the Athenians abandoned his strategy and embarked upon a less passive policy, designed by Cleon, and started to attack Sparta at home. After ten years of war, the Spartans were forced to admit that they had been unable to defeat Athens, and signed the Peace of Nicias (421).
Now, Athens started an increasingly aggressive policy. Alcibiades convinced his compatriots that they had to join a new anti-Spartan alliance, which ended in disaster (battle of Mantineia, 418). Not outdone by this outcome, Alcibiades proposed to send an armada to conquer Sicily. Many people now thought that the extravagant politician should leave the city, but after the decision to organize an ostracism had been taken, he found the support of Nicias, and together they were able to obtain more votes for someone else, Hyperbolus, a radical democrat.
Alcibiades was made one of the three commanders of the Sicilian Expedition (the others were Nicias and Lamachus). Shortly after their arrival, and after they had agreed to pursue Alcibiades' tactics, he was recalled because he was believed to have been involved in a religious scandal (415). The expedition to Sicily ended in disaster too.
Understanding that his life was in danger, Alcibiades went into exile in Sparta, where -according to our sources- he convinced the authorities to start the war against Athens anew (the Decelean or Ionian War). The moment was well-chosen, because in 413 the Athenians had supported Amorges, a rebel in the Persian empire. Almost immediately, the Persians sided with Sparta. This was to be Athens' undoing. It could overcome the loss of the Sicilian expedition force, but could not fight against Sparta and Persia at the same time.
Ironically, Alcibiades was able to return to Athens after he had made a false promise to forge an alliance between Persia and Athens, but he had to leave his home town when it became clear that he could not keep his word. Alcibiades went into exile again.
After the battle at the Aigospotamoi, Athens was forced to surrender (404); Alcibiades was killed almost immediately after. A couple of years later, the Athenians avenged themselves upon his teacher, Socrates, who was forced to drink hemlock.
Plutarch's Life of Alcibiades is available at LacusCurtius.