The Peloponnesian war began in 431 BC and had as its main rivals Athens and Sparta and ended in 404 BC, forcing Athens into a humiliating defeat It was a catastrophic war, not only for the defeated party but also for the victorious one, as it considerably weakened the city of Sparta and empowered other Greek cities to come to the forefront.
Internal changes were made during the same period, thus affecting the city¢s political system, such as the sale of lots of ground allotted to each citizen, that resulted in large portions of land being concentrated in the hands of few wealthy Spartan landholders. The passing of the new legislation that made such changes possible was followed by a decrease in the number of equals, namely the free men, which accounts for the lack of sufficient male population and the subsequent acceptance of the ¡perioikoi¢ and ¡helots¢ as Spartan citizens.
In 371 BC the Thebans defeated the Spartans at the battle of Lefktra, thus bringing down the myth of the invincible Spartan army and gradually curtailing its power.