The Greeks were able to regroup. To defend against the Persians, Athens formed the Delian League, making the city-state the most powerful in Greece. In response to the growing power of Athens, the Spartans formed their own league, and the two powers went to war. At the end of this war, Greece was extremely weakened, yet the Persians never attempted another conquest. However, one power did: the Macedonians, who were able to unite most of Greece under one banner and then conquer the Persian Empire.