What Happened to Athens After the Peloponnesian War?
As the loser of the Peloponnesian War, Athens was fundamentally changed by the conflict. Indeed, it entered the war as one of the two main powers in Greece, but ended it under Spartan subjection. This was followed by a brief, yet bloody period of rule by an oligarchy known as the Thirty Tyrants, which contrasted with Athens' traditionally democratic system of government. While democracy was ultimately restored, Athens nonetheless continued to decline in power in the years following the Peloponnesian War.
Background

At the beginning of the war, Athens was arguably the most powerful city-state in Greece. This was largely the case due to the Delian League. Established as an alliance of city-states to help defend against the Persians, by the 440s and 430s BCE, it essentially functioned as an Athenian Empire, with Athens collecting tributes from members in the form of monetary payments and ships. This strengthened its military and threatened the traditional dominant power in Greece, Sparta. Athens further challenged Spartan power due to its democratic system, since the oligarchic Sparta feared that its allies would revolt to become democratic. When combined with more immediate flashpoints between Spartan and Athenian allies, war finally broke out between the two city-states in 431 BCE.
Athens made a series of military and political errors throughout the conflict that greatly weakened its position. Perhaps the most consequential was the Sicilian Campaign, which culminated in the Battle of Syracuse in 413 BCE. Hundreds of Athenian ships were destroyed, and thousands of sailors were killed. Athens was further weakened after the Battle of Arginusae (406 BCE), when its citizens voted to have six key generals executed for failing to rescue thousands of sailors in the water. Then, Athens suffered a decisive defeat at the Battle of Aegospotami in 405 BCE, where its fleet was destroyed. With its navy weakened and military leadership decapitated, Athens finally surrendered in 404 BCE and was occupied by Sparta.
The Thirty Tyrants

Sparta subsequently replaced the traditional democratic Athenian government with a group of oligarchs. Led by figures like Critias and Theramenes, these oligarchs were known as the Thirty Tyrants as they led a regime of repression and violence. Mass executions were common, with hundreds (and possibly thousands) being killed. Wealthy Athenians also often had their assets targeted and seized, and the violence reached the highest levels of Athenian society. All this resulted in many fleeing the city. Indeed, Theramenes, a relative moderate, opposed the extremes of the regime, which Critas responded to by having him executed. While this demonstrated the brutality of the government, it was also indicative of its instability. When combined with organised resistance from democratic exiles like Thrasybulus, the Thirty Tyrants fell in 403 BCE after only eight months in power.
The Return of Democracy

While Athenian democracy ultimately returned, it was somewhat different than before and during the Peloponnesian War. Many still feared the influence of provocative public speakers and unchecked rhetoric on impulsive decisions, such as the aforementioned execution of the six generals after the Battle of Arginusae. While this resulted in a society that was more based on the rule of law, it also contributed to a generally heightened suspicion toward individuals associated with anti-democratic ideas. Such an atmosphere helps explain the trial and execution of famed philosopher Socrates in 399 BCE, who was found guilty on charges of impiety and corrupting the youth. In short, post-war Athenian democracy was characterized by a more critical and cynical nature.
Continued Decline

Despite the restoration of democracy, Athens never again returned to the strength it had enjoyed before the Peloponnesian War. The loss of the Delian League meant that its military and economic capacity was significantly weakened. Athens did regain some power in the Corinthian War from 395 BCE to 387 BCE, and again with the complete decline of Spartan hegemony after the Battle of Leuctra in 371 BCE. Nonetheless, the overall weaknesses of most Greek city-states meant that Athens was never able to re-establish the alliances that had made it so powerful at its peak, despite founding the Second Athenian League in 378 BCE.
Furthermore, to the north in Macedonia, its leader, Phillip II, was making increasingly imperialistic overtures. This ultimately culminated in the Battle of Chaeronea in 338 BCE, when Athens (along with its ally, Thebes) was beaten by the Macedonians. After the battle, Phillip II formed the Corinthian League, marking the beginning of nearly 200 years of Macedonian hegemony in Greece and the effective end of Athenian independence.
Impact and Legacy

Athens experienced significant difficulties after the Peloponnesian War. Sparta immediately installed an oligarchy known as the Thirty Tyrants, which was responsible for widespread bloodshed. Despite the almost immediate restoration of democracy, it was far more restrictive and less open to new ideas than before the war. Finally, the loss of the Delian League, paired with the weakness of other Greek states, meant that Athens was unable to establish the same sort of alliances that it had at the height of its power, thereby paving the way for a state like Macedonia to take control of all of Greece.