The Messenian War
As the population of Greece was growing many states began to establish colonies. Sparta founded only one colony, this was Tarentum in southern Italy.
Sparta preferred to expand against its neighbor Messenia in the western Pelopennes. An attack by Sparta forced the Messenians into their mountain retreat Ithome. They fought there for twenty years after which the defenders fled from the mountains.
After the subjection of the Messenians Sparta developed into a military state.