in 431 BC.]]The Delian League was an association of fifth-century BC Greek city-states (approximately 150) whose purpose was to continue fighting Persia after the Greek victory in the Battle of Plataea. According to Thucydides (1.96), the official aim of the League was to "avenge the wrongs they suffered by ravaging the territory of the king." In reality, this goal was divided into three main efforts - to prepare for future invasion, to seek revenge against Persia, and to organize a means of divvying spoils. League members swore to have the same friends and enemies, and dropped ingots of iron into the sea to symbolize the permanence of their alliance.
In 478 BCE, following the defeat of Xerxes' invasion of Greece, Pausanias the Spartan led Hellenic forces against the Persians. He was an unpopular commander (who may have conspired with the Persians), and although he was cleared of all accusations of conspiracy, Sparta, eager to stop prosecuting the war, decide
Source: http://en.wikipedia.org/wiki/Delian League