The Battle of Thermopylae, the battle where Leonidas, 300 Spartans, and a couple of thousand other – usually forgotten Greeks – fought, is one of the most famous battles in all of history. And when it comes to the battle the most common question is why Leonidas didn`t retreat.
Aside from that question, there are two other extremely common questions. Why did the Greeks fight at Thermopylae? And what was the Greek plan for the Battle of Thermopylae? Both questions will be answered in the following.
Greece has three natural lines of defense (the Vale of Tempe, the passage of Thermopylae, and the Isthmus of Corinth) against an invasion from the North. But defending at the Thermopylae was the only real option since the Vale of Tempe could easily be bypassed and defending at the Isthmus of Corinth would have left mainland Greece (including Athens) to the Persians. The Plan at Thermopylae wasn`t to defeat the Persian army. The plan was to buy time so that the Greek fleet in the Straits of Artemisium could defeat the Persian navy, which would have forced the Persian army to retreat.
Let`s take a closer look and start by looking at why the Greeks decided to fight at the passage of Thermopylae.
Why did the Greeks fight at Thermopylae?
The Greco-Persian Wars reached a new level of intensity when the long-prepared invasion of Greece started in 480 BC (the Persian attack in 492 BC that was halted at the Battle of Marathon in 490 BC was not an invasion of Greece, but punishment for the city-states of Athens and Eretria since their support for an ant-persian rebellion had started the Greco-Persian Wars in the first place).
So while the Persian attack of 492 – 490 BC was not an invasion of Greece but revenge against the Greek city-states of Athens and Eretria (both had supported an Anti-Persian rebellion (the Ionian Revolt) in Asia Minor from 499 – 493 BC), the second attack on Greece (480-479 BC) targeted all of Greece.
As a response to the Persian threat, 31 Greek city-states from Central and Southern Greece founded the Hellenic League, a defense alliance against the Persians. By the way. While it is often said that all of Greece united to fight the Persians, that is absolutely wrong! There were more than 1,000 Greek city-states sprinkled all over the Mediterranean but mostly in Greece and Asia Minor. But only 31 of these more than 1,000 Greek city-states joined the fight against the Persians, the rest remained either neutral or fought on the Persian side.
Especially the Persian navy included large numbers of Greek ships, crews, and officers!
The Hellenic League, the anti-persian alliance of 31 Greek city-states from Central- and Southern Greece now had to decide where they should fight the Persians and stop their advance.
There were 3 options.
Greece has three natural lines of defense. The Vale of Tempe, a narrow gorge between the regions of Macedonia and Thessaly. The Thermopylae, a narrow (in parts only 100 m / 323 ft wide) coastal passage between the regions of Thessaly and Central Greece. And the Isthmus of Corinth, a narrow land bridge that connected mainland Greece to the Peloponnese (that was controlled by Sparta).
These three lines were all possibilities for a defense against the Persian invasion. But on a closer look only one, the passage of Thermopylae, was suitable. Both the Vale of Tempe and the Isthmus of Corinth had major disadvantages that excluded them from being chosen as the site where the Hellenic League would try to stop the Persian advance.
The Vale of Tempe was the most Northern possible line of defense but it could easily be bypassed by the Persians. So the Vale of Tempe was ill-suited. The Isthmus of Corinth was the most Southern line of defense. But only facing the Persians at the Isthmus of Corinth would have meant that all of Central Greece – remember, many members of the Hellenic League were from Central Greece – would have been left to the Persians.
And let`s face it. It would have been extremely difficult to convince the soldiers from the Central Greek members of the Hellenic League to fight the Persians at the Isthmus of Corinth while the Hellenic League had left their homes to Persian destruction without a fight.
So only the passage of Thermopylae remained as a suitable line of defense!
By the way, while the Persians had meticulously prepared their invasion the same can not be said for the Hellenic League. One example of the lack of Greek organization can be seen in the deployment of 10,000 Hoplites to the Vale of Tempe.
Weeks before the Battle of Thermopylae (that started on 19 August 480 BC) 10,000 Hoplites were sent to the Vale of Tempe with the order to defend the canyon against the advancing Persians. But as soon as they arrived they realized that the Vale of Tempe could easily be bypassed. So they went back onto their ships (250 triremes) and sailed to the Isthmus of Corinth.
10,000 Hoplites was a huge army, more on the size of Greek armies in my article here, especially compared to the 7,000 men who then fought at the Battle of Thermopylae.
So what was the plan for the Battle of Thermopylae? And why was the Greek army at the Thermopylae that small compared to the force that had been deployed to the Vale of Tempe?
The Greek plan for the Battle of Thermopylae
Aside from its position north of the lands of the members of the Hellenic League the passage of Thermopylae also had another huge advantage: It was pretty narrow.
At its narrowest points, the passage of Thermopylae was only 65 – 98 ft (20-30 meters) wide, which offset the huge numerical advantage of the Persian army.
It was clear that despite the geographical advantage, the Greek army would not be able to defeat the Persian army in the Battle of Thermopylae. But that was also not the goal. The goal was to buy time for the Greek navy to win a victory over the Persian fleet in the simultaneous naval Battle of Artemisium (both battles started on 19 August 480 BC).
So the Greek hopes rested on the navy that mostly consisted of Athenian Triremes. While the Greek navy was still numerically inferior to the Persian navy (which mostly consisted of Greek and Egyptian ships), both navies were still much closer in numbers since a storm had decimated the Persian navy.
Well, as I said, the numerical differences between Greeks and Persians were much smaller in the navies. So the chances for success were much better.
But there was one more point.
The Persian navy and the Persian army closely worked together. So when one of the two was defeated, then the other one would also have to retreat. The Greeks hoped that defeating the Persian navy (an undertaking with much greater chances of success than trying to defeat the Persian army), would force the Persian army to withdraw from Greece without being defeated in battle itself.
Now that might sound odd at first. Why were the Persian navy and army so dependent on each other?
Well, the Persian navy did not only cover the rear of the Persian army from potential landings and sea-based attacks by the Greeks, but the Persian navy was also crucial for supplying the Persian army. Vice versa the Persian army covered the Persian fleet from land-based attacks against it when the ships landed for repair or rest.
So: The Greek plan for the Battle of Thermopylae was that the land-based army under the command of Leonidas would halt the Persian army until the Greek navy, which was waiting in the straits of Artemisium, could defeat the Persian fleet. Without the cover and support of its navy, the Persian army would be forced to retreat without being defeated in battle.
The plan was good but it had one big weakness: It relied on the Greek navy to find an opportunity to defeat the Persian navy. And while the Greek navy tried, it could not do that.
And soon the land-based forces came under pressure when the Persians found a way to bypass the passage of Thermopylae with the help of a local guide).
Suddenly the army under the command of Leonidas was at risk of being encircled. But not only that, the only way for the Greek fleet to escape from the Straits of Artemisium south towards safety was to pass through a narrow, only 50 ft (15 meters) wide fairway close to the passage of Thermopylae.
In order to ensure that the Greek navy could safely pass through that fairway the Thermopylae had to be held for as long as possible.
And that brings us to the question of why Leonidas and his Spartans didn`t retreat… I actually wrote an entire article on that topic so please check it out!
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Until next time
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