What historical event left the Greeks weak and divided?

The historical event that left the Greeks weak and divided was the Peloponnesian War (431-404 BCE), a protracted conflict between the city-states of Athens and Sparta, along with their respective allies. The war concluded with the defeat of Athens, leading to significant political instability, economic hardship, and social strife across the Greek city-states. This fragmentation weakened their unity, making them more vulnerable to external threats, notably the rise of Macedon under Philip II.