By the early fifth century BCE, Sparta was the leading military power of Greece. It headed an alliance of Peloponnesian cities that modern historians call the Peloponnesian League. But to the east rose a power far larger than any Greek city: the Persian Empire.

The Persian Empire

Founded by Cyrus the Great in the mid sixth century BCE, the Achaemenid Persian Empire stretched from Egypt and Anatolia to Central Asia and the borders of India. It was the largest empire the world had yet seen, with tens of millions of subjects, a sophisticated administration, a network of roads and a tradition of allowing conquered peoples to keep their customs and religions, as long as they paid tribute and stayed loyal. Greek cities in Ionia, on the western coast of Anatolia, had come under Persian rule.

The Ionian Revolt and Marathon

In 499 BCE, the Ionian Greek cities revolted against Persia. Athens and Eretria sent ships to help; Sparta refused. The rebels burned the Persian provincial capital of Sardis before the revolt was crushed. King Darius I resolved to punish Athens and Eretria.

In 490 BCE, a Persian expedition destroyed Eretria and landed at Marathon, northeast of Athens. The Athenians sent a runner, Pheidippides, to Sparta to ask for help, covering about 150 miles in two days, according to Herodotus. The Spartans agreed to help but said they could not march until the full moon, because they were celebrating a religious festival, the Carneia. By the time they arrived, the battle was over: the Athenians and their Plataean allies had defeated the Persians at Marathon on their own. Spartan delays for religious festivals would happen again, and they show how seriously Spartans took their obligations to the gods.

Xerxes

Darius died in 486 BCE before he could launch another invasion. His son Xerxes prepared a far larger one. Persian engineers built a bridge of boats across the Hellespont, the strait between Asia and Europe, and dug a canal through the peninsula of Mount Athos to avoid the stormy cape where an earlier Persian fleet had been wrecked. In 480 BCE, Xerxes crossed into Europe with an army and fleet of enormous size.

How big was Xerxes' army?

Herodotus claimed that Xerxes' forces, including support personnel, numbered over five million people, with more than 2.6 million fighting men. Modern historians consider this impossible: no ancient army of that size could have been fed or supplied. Estimates vary widely, but many historians think the Persian land army numbered perhaps one hundred thousand to two hundred thousand men or more, still far larger than any force the Greeks could field. The fleet probably numbered several hundred to over a thousand ships.

Greek responses

Many Greek cities, especially in the north, submitted to Persia, offering the traditional symbols of earth and water. Others decided to resist. In 481 BCE, representatives of these cities met and formed an alliance under Spartan leadership, often called the Hellenic League. Sparta held overall command on land and, at first, at sea, even though Athens supplied the largest fleet. The Athenian leader Themistocles had persuaded his city to use a rich new silver strike to build a fleet of warships, a decision that would prove decisive.

An oracle

Herodotus reports that the Spartans consulted the oracle at Delphi and received a grim prophecy: either Sparta would be destroyed by the Persians, or Sparta would mourn the death of a king descended from Heracles. Herodotus suggests that King Leonidas knew of this prophecy when he marched north. Whether the oracle was given before the battle or shaped afterward, it became part of the story of his sacrifice.

Choosing a defense

The Greek allies needed a place to stop the Persian army. Northern Greece was flat and open, ideal for Persia's numerous cavalry. Farther south, at a narrow pass between mountains and sea called Thermopylae, meaning “hot gates” after nearby hot springs, a small force might hold a much larger one. At the same time, the Greek fleet would block the Persian navy at nearby Artemisium, so the Persians could not land troops behind the pass.

Worked example: Persia from the other side

Greek sources, especially Herodotus, are our main account of the Persian Wars. They naturally present the Greeks as defenders of freedom against a despot. Try to see the war from the Persian side.

• The empire's view. Athens had helped burn Sardis, a Persian city. Punishing rebels and their supporters was what any empire would do.
• Persian rule. Persian kings often allowed subject peoples their own religions and local rulers. The Cyrus Cylinder, a Babylonian text, presents Cyrus as a liberator who restored temples. Many Greeks served Persia willingly.
• Greek divisions. Many Greek cities, including powerful Thebes, sided with Persia or stayed neutral. “The Greeks” were not united.
• The real stakes. For Athens, defeat would likely have meant destruction and loss of its new democracy. For Sparta, it might have meant submission and tribute.

Herodotus himself, to his credit, often treated Persians with respect and curiosity. Seeing a conflict from more than one side does not mean every side was equally right. It means understanding why people acted as they did, which is the first step toward judging fairly.