First, there were numerous entrances into northern Hellas and the broad plain of horse-rearing Thessaly. The most important of these was the Vale of Tempe, a five-mile-long defile, but it would be hard to halt the Persian advance there because it was easy to turn this defensive position by passing through another access point not far away. Unsurprisingly, most of the states in the area did not look forward to armies fighting on their own land. They were minded to capitulate to the Great King and medize.
Then, leading out of Thessaly and into central Greece was a narrow pass between the sea and mountains at Thermopylae (“hot gateways,” so-called because of hot springs). The waters were narrow too at Artemisium, northern cape of the island of Euboea. The two places had the signal advantage of being close enough to one another for reasonably speedy intercommunication (a distance of about forty miles by water).
And, third, a last-ditch defense could be mounted at the Isthmus of Corinth between mainland Greece and the Peloponnese peninsula. It was four miles wide and could be fortified.
Any commander planning how to beat the Persians needed to bear some factors, both positive and negative, in mind. The Great King’s men needed food and drink and, of course, had not been able to establish food dumps on enemy territory. However, so long as he maintained command of the seas, transport ships could bring in regular supplies. If anything were to happen to the fleet, the army would find itself in serious difficulties. Xerxes would want to complete a conquest as quickly as possible. If the Greeks could hold up the invaders for long enough they might be able to force a Persian withdrawal.
The Persian fleet was keeping pace with its ground forces. Unless it could be stopped from doing so, it could sail ahead and land troops in the rear of the Greek military positions. Defending successfully the Isthmus of Corinth would do no good if the Persians could simply sail past it and open a bridgehead on the Peloponnese, perhaps from a base on the island of Cythera off its southern coast. On the other hand, there were few large harbors on the peninsula and the Persians were unfamiliar with the lie of the land. We must never forget that people knew little of geography in those days of slow, uncomfortable, and sometimes dangerous travel.
One final consideration: a brand-new Athenian fleet alongside those of other Greek poleis could now put up a good show against the Persians. About forty thousand men were needed to man the city’s two hundred triremes. The number of adult male Athenians at this time has been estimated at between forty and sixty thousand (probably at this period nearer the former). Most oarsmen were thetes, members of the lowest socioeconomic class. There were also about 25,000 metics, or long-term foreign residents, who could be conscripted. Both citizens and metics underwent intensive training. Manning all the boats at once was feasible, but placed a serious strain on Athenian manpower.
However, the Greeks were still outnumbered and their warships were heavier and less maneuverable than their Phoenician counterparts. These were good reasons for avoiding battle in open waters. There were only two stretches of sea where it would be safe to fight—Artemisium, as already mentioned, and in the cramped waters between the coast of Attica and the island of Salamis.
It was all too easy to scare oneself into a defensive frame of mind. If the Persian threat was to be eliminated, the question was rather how to win the war than how to avoid losing it. It was to this that the most imaginative political and military mind of the day was looking for an answer. It was the mind of Themistocles.
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Sparta, by common consent the leading polis of the Hellenes and unmatched on the battlefield, and Athens, the victor of Marathon, convened what they called a Congress of Representatives at Corinth. It met in the autumn of 481 while Xerxes was still training his forces at Sardis. Sparta was in the chair and thirty-one states attended. The Congress’s task was to decide what measures to adopt for a common resistance to the invader. It was an exceptional occasion, for it was almost unheard of for Greek city-states to try to agree on something, or indeed on anything, but without at least a semblance of unity they had no hope of success.
The allies announced an end to their endemic mutual feuds, and in November Athens and Aegina abandoned their on-off, low-level hostilities. Themistocles had a main hand in this general accord, for which Plutarch rightly gives him generous credit:
The greatest of all his achievements was his putting a stop to the wars among Greeks and reconciling Greek city-states with one another. He persuaded them to postpone their differences because of the war with Persia.
We will not go far wrong in also attributing to Themistocles a decision of the Athenian ecclesia to recall all the ostracism exiles: Aristides and Xanthippus were patriots and were to play leading parts in the approaching struggle. Reconciliation abroad was to be matched by harmony at home.
The Congress sent spies to investigate the Great King’s preparations in Asia Minor. They were caught, but, instead of executing them, Xerxes astutely had them shown around his camp so that they could report back the huge scale of the expeditionary force (apparently they did not see the fleet, which meant that the Greeks were much less well informed about Xerxes’ naval strength).
Persian ambassadors were doing their best to persuade Greek city-states to submit in advance to the Great King. In response, the allies voted to confiscate the territories of all that did not join the struggle for survival. They sent their own envoys to bring around those that were reluctant to join an anti-Persian coalition, among them the Thessalians, Argos, Sparta’s longtime bête noire in the Peloponnese, and Thebes together with the other cities in Boeotia.
Appeals for assistance were sent to Crete, Corcyra, and the rich and powerful city-state of Syracuse, which dominated eastern Sicily. Nothing came of these initiatives, and it may well be that Xerxes entered into a pact with Syracuse’s archenemy, Carthage, which was planning an invasion of Sicily. The idea was that Syracuse would come under attack at the same time as the Great King invaded mainland Greece, so preventing it from sending help.
The Congress met again in the spring of the following year. It was time to decide the structure of command in the coalition forces. Everybody accepted that Sparta should have supreme command of the army, but Athens, now the largest Greek naval power, had hoped to be given command of the navy. However, other delegates threatened to leave the coalition if they had to serve under an Athenian. Themistocles, whom the Athenian ecclesia had elected as general (typically, he bribed another likely candidate to stand down at the elections in February), waived his city’s claim in the larger Hellenic interest. He accepted a Spartan, Eurybiades, as overall commander of the allied fleet. Herodotus wrote that the Athenians gave way from the best of motives:
They considered the survival of Hellas to be of supreme importance and, if they quarrelled over the leadership, Greece would face destruction. They were absolutely right. Just as war is worse than peace, so civil strife is much worse than a united war effort.
Spartans saw themselves as a land power and knew next to nothing about naval matters. However, as the historian Diodorus Siculus put it, Eurybiades was in charge, but Themistocles gave the orders.
The northern states made it clear that they would be forced to medize unless Sparta sent some troops to defend the Vale of Tempe. Before the Great King had crossed from Asia to Europe, a force of ten thousand was duly dispatched, but it stayed only a few days. Locals were unhelpful.