Remember at the beginning, when Athens and Sparta united to defeat the Persians? They succeeded, but that didn't make them great pals. Once Greece's two most powerful city-states had beaten their common enemy, their own rivalry resumed. This led to a long string of battles called the Peloponnesian War.
How did the war start? Athens's leaders wanted more power over other city-states. They made the Delian League into their own Athenian Empire. Athens became very wealthy from the tributes, or taxes, it collected from other city-states. Pericles used a lot of the money to strengthen Athens's defense walls (known as the Long Walls). They protected the route between Athens and its port, Piraeus, about seven miles away. Pericles also built up the Athenian fleet, already the biggest navy around. Athens used its powerful navy to pressure more city-states to accept its rule.
