Greece was not unified - it remained as several hundred independent city-states, some of which banded together in leagues which fought each other and changed members from time to time.
persians
Two leagues - one led by Athens, the other led by Sparta.
The Persian failed invasion of mainland Greece 480 to 479 BCE.
Darius
Xerxes
After the Persian Empire gave up and agreed to peace, Athens converted the Delian League, which it had led against Persia in the latter stage of the war, into an empire of its own.
Xerxes
He led an unsuccessful invasion of mainland Greece 480-479 BCE.
King Xerxes I led the expedition to Greece in 480-479 which was driven back by the combined forces of the southern Greek city-states.
The Delian League led by Athens and the Peloponnesian League led by Athens.
You'd have to be more specific about when, but the most likely answer is the Persian Empire, led by Darius III.
Germany's unification led to the collapse of the Soviet union.