Decelea, modern Dekeleia or Dekelia, Deceleia or Decelia, previous name Tatoi was a decisive source of supplies for Athens. With advise from Alcibiades, former Athenian General who was wanted on charges of religious crimes, Spartans fortified Decelea in the Peloponnesian War preventing Athens from getting supplied by land. This decimated Athens, who was already getting badly beat in the Sicilian Expedition.
Decelea is situated in a pass in the Parnes range, near the source of the Cephisus river, and -according to the Athenian historian Thucydides- 120 stadia (21 kilometer) north of Athens.
In the spring of 413, the Spartan king Agis II occupied and fortified Decelea, an act of strategic and symbolic importance.
The capture and fortification of Decelea marked the beginning of the second phase of the Peloponnesian War, which is called the Decelean War.