The Peloponnesian War between Athens and Sparta consists of two parts: the Archidamian War (431-421) and the Decelean or Ionian War (413-404). When the first part had ended in an Athenian victory, the Athenians believed they could now try to expand their empire. A large expedition to Sicily was believed to be the best idea, and in 415 a large armada left Greece and went to the west. Over there, everything that could go wrong, went wrong. The Sicilian Expedition ended in failure; of the many who had left, only a few a returned.
Benefitting from the absence of a great part of the Athenian forces, the Spartans decided to renew the Peloponnesian War (413). This time, they were more successful: in 404, Athens surrendered. Nearly always, the Spartan victory is explained from the demise of Athenian power after the expedition to Sicily, but this cannot be correct. In fact, Athens recovered quickly, and in 410, the Spartans regretted their declaration of war. They offered peace negotiations, but Athens declined: after all, Sparta had twice broken a treaty (in 431 and in 413), and Athens demanded some kind of guarantee that the Spartans would leave the Athenians in peace. When Sparta was unwilling to offer this, fighting was resumed.
In the meantime, however, the Persian king Darius II Nothus had decided to support the Spartans. Now, Sparta had the money to create a navy, and although it still suffered several defeats in naval battles, in the end, it was victorious at Aigospotamoi (405). Athens no longer controlled the sea, and was starved into surrender.
What caused the fall of Athens? Not the Sicilian disaster: Athens recovered sufficiently to make the Spartans decide to negotiate again. It was Sparta’s Persian alliance that shifted the balance of power, so the ultimate question must be why the great king decided to abandon his policy of non-interference.
The answer to this question, and the deepest cause of the fall of Athens, is that the Athenians had supported one Amorges, a rebel in Asia Minor, fighting against the Persians. This was unacceptable to king Darius, who now decided to support Sparta. The orator Andocides explains (On the Peace 31-32):
The king’s runaway slave, Amorges, induced us to discard the powerful support of his master as worthless. We chose instead what we imagined to be a more advantageous understanding with Amorges himself. The king in his anger replied by allying himself with Sparta, and furnished her with 5,000 talents with which to prosecute the war; nor was he satisfied until he had overthrown our empire.
Her support of Amorges, and nothing else, lost Athens the war.