The Ionian or Decelean War (413–404 BCE) was a significant conflict during the Peloponnesian War between the Athenian Empire and the Spartan alliance. It marked a shift in the war's dynamics, as Sparta received support from the Persian Empire, allowing them to challenge Athenian naval supremacy. This phase of the war is characterized by the establishment of a Spartan base at Decelea in Attica, which disrupted Athenian resources and morale. The conflict ultimately led to the decline of Athenian power and the eventual defeat of Athens, reshaping the political landscape of ancient Greece.