Even though the Dark Age ended and civilization returned to Greece, the Greeks were still NOT united. In fact, the 2 largest city-states, Athens and Sparta, HATED each other with a passion. However, just as in the past, there was still one thing that could unite them a common ENEMY.
After the Dorian invasion, many Greeks settled in Ionia in the southern part of Asia Minor. By 499 BC, Asia Minor was part of the ever expanding Persian Empire. Ionian Greeks were MAD. They revolted against the Persians. Some Greek citystates (like Athens) sent help, but the Persians crushed the revolt.
King Darius I
King Darius I was FURIOUS at the Athenians. He vowed to get EVEN. At each meal, a slave whispered 3 times to the king: "Sire, remember the Athenians." His anger grew each day. In 490 BC, Darius sent a fleet of ships to Greece. They attacked and burned the city-state of Eretria, just north of Athens.
Alarmed and fearing attack, Athenian leaders sent a herald (messenger) named Pheidippides to Sparta seeking their help. He ran the 150 miles to Sparta in TWO days. Sparta agreed but due to an unusual religious tradition ( waiting for a full moon before going into battle) Spartan soldiers did not arrive in time to help.