The Athenian Empire began to decline after the Greek League defeated the Persian Empire. The Sparta-led Peloponnesian League challenged its ascension, defeating Athens after almost a century of war.
Athens adapted its culture and military to use some of the good tactical ideas and technologies of the Persian Empire, especially its powerful navy. With this, it partnered with other Greek city-states to form the Delian League. More and more Athenians were drawn to live in the city, eliminating much of the farmer/hoplite pool of soldiers from which to draw. Instead, Athens created a standing army, supplementing it with mercenaries from the new colonies it created throughout the Mediterranean region.
Alarmed, Sparta joined with the other Greek city-states to form the Peloponnesian League, intending to counter Athens. The two sides fought several wars between 431 and 338 BC, over time weakening all the Greeks. Ultimately, Athens lost most of its colonies and hence most of its wealth. With Athenian ambitions defeated, Sparta became the dominant culture in Greece, and Athens began seeking a different kind of domination. Athens' golden age of philosophy began at the moment the Athenian empire fell.
Unfortunately for the Greeks, the Macedonians took advantage of the Greek's new weakness. Creeping south, Philip II of Macedonia started with diplomacy and shifted to war in order to take over Greece one city at a time, ultimately uniting it as Athens could not. He hired the Athenian philosopher Aristotle to teach his young son and heir Alexander, who grew up to be Alexander the Great.