Ancient Greece, Middle Classical Period, Second (=Great) Peloponnesian War, Thirty Years’ Treaty (446-431 BC): Incident at Plataea, Pentecontaetia 

Incident at Plataea (431 BC)

A state of war between Sparta and Athens had yet to be created. In spring 431 BC the gates of Plataea were opened after dark by a private citizen to admit three hundred hoplites sent by Thebes, but the force was inadequate and was overpowered by the Plataeans. 

The main Theban force, which had been delayed, was met by a Plataean herald who promised that the prisoners would be returned. But when the Theban force withdrew and the people had been collected safely inside the city walls, the Plataeans slaughtered all one hundred and eighty of their Theban captives. Both Athens and Sparta considered this an act of war as the two participants had broken the treaty by committing hostilities against one another.

Pentecontaetia (‘the period of fifty years’)

The term ‘Pentecontaetia’ was originated by Thucydides as a convenient, though slightly inaccurate, description of the period between the battles of Plataea and Mycale (479 BC) and the outbreak of the Second Peloponnesian War (431 BC). The period 479-461 BC can be spoken of as the ‘Age of Cimon’, and the period 461-429 BC as the ‘Age of Pericles’.