A sudden outbreak of plague that destroyed 2/3's of the Athenian population and a soon following defeat against Sparta's ally Syria in war. (What do you know you really DO use world history in real life)
Addendum
The plague was a factor but the two thirds loss is questionable. And even after that setback, they did rather well for the next 20 years of the war. A more serious loss was the early death of Pericles which deprived them of his steadying hand, and left the democracy to being swayed the erratic demagogues.
Other factors were:
1. Athenian adventurousness - their foray in Sicily against Syracuse cost them dearly in manpower, ships and money, seriously weakening their military capacity.
2. Persian financial support to the Spartan alliancewhich enabled them to assemble a fleet which could match the hitherto dominant Athenian fleet.
Athens' allies were either defeated or neutralised. Isolated Athens was short of manpower and money without its empire.
Persia began to provide finance for the Spartan confederation to upgrade its fleet to match the hitherto superior Athenian fleet, which was defeated at Aigospotomai, Athens was invested and starved into surrender.
During the Peloponnesian war.
After 27 years Athens, its fleet destroyed, was besieged and starved into surrender.
Lysander.
Persia provided the funds for the Peloponnesian League to build and man a war fleet to match the hitherto superior fleet of Athens.
Athens suffered a number of disasters: the plague, Syracuse, and the defeat of its fleet.
Athens and its empire by Sparta and its allies, with financial support from Persia.
Not good enough to avoid defeat by Athens at the start of the Peloponnesian War.
The Peloponnesian War.The Peloponisian War
Athens lost the Peloponnesian War because it refused to agree to peace on reasonable term, and the Persians bankrolled the Peloponnesian League to form a navy strong enough to defeat the hitherto superior Athenian fleet.
The Peloponnesian War was between Athens and Sparta. It was after the great Persian war, when Athens and Sparta worked together to defeat Persia. The Peloponnesian War was started because Sparta feared Athens may dominate them one day. There was a big battle between them, lasting for many years, but in the end, Sparta won.
After its defeat in the Peloponnesian War it was stripped of its empire and became a second rate power and had to live on its own means.
The Peloponnesian War of the Athenian empire versus the Peloponnesian League led by Sparta resulted in the defeat of Athens which was stripped of his empire. Continuing war between Greek city-states enabled Macedonia to move in and take over.