Peloponnesian War: Athens has been fighting Sparta for supremacy in both Greece and the wider Aegean since 431 BCE, but on this day Athens is forced to surrender. Before the Peloponnesian War Athens was the strongest Greek city-state and led an empire; after the war Athens is reduced to complete subjugation.
Sparta becomes the leading power of Greece, but it's not interested in widespread trade and economic development. Thus Greece suffers a double hit: there are massive costs from the war itself, and the post-war economy shrinks due to a decline in economic activity.
A further consequence is the damage done to the democratic system in Athens. Corinth and Thebes want Athens destroyed and all its inhabitants enslaved, but Sparta refuses and takes responsibility for Athens thereafter.