In the second half of the 6th century, Athens fell under the tyranny of Peisistratos and then of his sons Hippias and Hipparchos. However, in 510 BC, at the instigation of the Athenian aristocrat Cleisthenes, the Spartan king Cleomenes I helped the Athenians overthrow the tyranny. Afterwards, Sparta and Athens promptly turned on each other, at which point Cleomenes I installed Isagoras as a pro-Spartan archon.