Ancient Greece was not a united country with one king or leader. Instead, it was made up of lots of city-states, each with its own ruler. There was always a large city at the centre of a city-state. Each city-state had formed its own government, and some had kings as well. The two most powerful city-states were Athens and Sparta.
Sparta is well-known for its military strength. It had a very strong army, because Spartan boys trained to become warriors from the time they were 7 years old. They joined a tough military school where they learnt how to become a mighty warrior.