Thucydides was an Athenian aristocrat  who was in his late twenties when the conflict that he would call the Peloponnesian War began in 431. He was keenly aware of the significance of the conflict and began writing its history almost immediately.
In 424, Thucydides was elected one of Athens’ generals, but his failure to prevent the loss of the city of Amphipolis to the Spartans saw him exiled from Athens. He spent the rest of the War wandering around Greece as a non-combatant gathering evidence and talking with participants from both sides for his history on the conflict.
Herodotus is regarded as the father of history, although Thucydides may perhaps more deserve the title. Whereas Herodotus, writing some 40 years earlier, was more interested in a ripping narrative and recorded almost everything he heard with little regard to truth or accuracy, Thucydides assembled evidence and compared verbal and eyewitness accounts and synthesized the evidence and then shaped it into something he believed to be truthful. It is also significant that the role of Gods or fate is dispensed with and the results of actions and events in the conflict are entirely due to human action and motives. He also details his motive in recording the events as accurately as possible so that future generations will get true picture of the war and personalities involved. Of course, as an Athenian and with a vested interest in the story, his History has its biases.
The growth of the Athenian commercial empire, which had grown steadily since the end of the Persian wars, had spread across the Aegean and disrupted the balance of power in Greece. Corinth, saw its extensive trade interests threatened by Athens's control of the seas and so many of the islands and ports of the Aegean and formed an informal alliance with Sparta to counter the power of Athens. The Greek world quickly split into two blocks, Athens and her Empire on one side, Sparta and her allies on the other. Both sides prepared for war, and it inevitably broke out in 431 over incidents in Corcyra and Potidaea (in northern Greece).
The conflict became known as the Peloponnesian War, and was fought all over the southern Greek mainland. The conflict lasted until 404, when Athens was finally defeated.

Thucydides details the causes of the War and gives an account of the diplomacy and debates that took place leading up to the War and the events of the first year. Perhaps the most famous passage of the history is the Funeral Oration of the Athenian leader Pericles who makes this long speech over the bodies of those who had died fighting for Athens. Pericles explains the nature of Athens's greatness, her freedom and democracy and explains why the sacrifice of the dead was worthwhile and was for a higher calling than just defence of home and family.
Thucydides describes the ideals that motivate the Athenians but is also critical enough of the war’s progress to detail the so-called The Melian Controversy, where the Athenians attacked the small island of Melos in 416, putting the city to the torch and killing and enslaving the people. Thucydides was the first to detail how this act, brought on by anger and frustration undermined the ideals the Athenians supposedly held so dear.
Thucydides then relates the account of the disastrous Athenian attempt to conquer the island of Sicily. One of the most startling scenes in the history is the story of a ragged man who walks into a barber shop in Athens and gets a shave and explains how he is all that is left of the great army that left two years earlier. The Sicilian disaster is one loss too many for Athens and her power quickly collapses and the end is inevitable.