Cleomenes I was a king of Sparta who reigned from 520 to 490 BCE. He is known for his military campaigns against Athens and Argos, as well as his efforts to reform the Spartan government. Cleomenes aimed to strengthen Sparta's power in the Peloponnesian region. During his reign, Cleomenes I played a significant role in the early conflicts leading up to the Greco-Persian Wars. He is also remembered for his controversial actions, including the expulsion of the Athenian tyrant Hippias. His reign ended with his eventual downfall and mental instability.