The Battle of Marathon -- part of the Greco-Persian Wars that the Greeks fought against Persian leaders Darius and Xerxes -- was an important day for the Athenians because they won despite Sparta's absence. During the Persian Wars the Spartans and Athenians were allies. This changed.
In 477 the Athenians and their allies set up the Delian League. Then came the battles between Athens and her allies and Sparta and hers that are known as the Peloponnesian War.