What historical event left the Greeks weak and divided?

Peloponnesian war

Explanation:

This war witnessed two warring Greek city-states Athens and Greece fight a long and bloody war which led to loss of many loves from both sides and shook the entire Greece from top to bottom.

The battle was caused by a breach of a peace treaty by the Athenians.

Sparta had the stronger land army, Athens had the better navy, although the Athenians were better battle prepared. It was a tough and long war that led to many deaths, alliances were formed, rebellions broke out, but many years down the line, the Spartans utterly defeated Athens with the aid of Persians.

This defeat was so total and devastating that it rendered the Greek military weak and divided.