Let's go back to the beginning. Remember when Athens and Sparta united to defeat the Persians? They won that war, but it didn't make them great pals. Once Greece's two strongest city-states had beaten their common enemy, their own rivalry started up again. This led to a series of battles called the Peloponnesian War.
How did the war start? Athens's leaders wanted to control other city-states. They turned the Delian League into their own Athenian Empire. Athens became very rich from the tributes, or taxes, it received. Pericles used a lot of that money to strengthen the city’s defense walls. Called the Long Walls, they protected the seven miles of road between Athens and its port, Piraeus. Pericles also built up the Athenian navy. It was the biggest fleet around. Athens used its big navy to make more city-states accept its rule.