The battle of Thermopylae was fought in the summer of 480 BC at the pass on the border between Malis and Phocis, where a steep mountain range reaches down to the coast, leaving only a narrow road between the slope and the sea.

The triple bottleneck of Thermopylae – the narrowest of which was only about a wagon’s width – has historically been the site of many attempts to block armies trying to march into Central Greece. However, a goat path known as the Anopea Path leads up the range and around the pass, and this has decided the outcome of every major battle here. The site is now unrecognisable because the sea has retreated about 2 km, leaving a wide coastal plain that would have been at best an impassable salt marsh in ancient times.

The Greek alliance led by Leonidas took up position behind a disused Phocian defensive wall and awaited the Persian attack. The battle lasted three days. On the first and second day, the Persians tried in vain to dislodge the Greeks by frontal assault. On the night of the second day, they sent the elite Immortals over the goat path to surround the Greeks in the pass. When the Greeks learned of this, most of them retreated, but Leonidas stayed behind with 300 Spartiates and some others, and all died in the ensuing last stand.

Despite some famous and often repeated numbers, we don’t actually know how many Greeks fought at Thermopylae. Our sources are not precise about the size of all contingents and their totals diverge pretty radically. There are some major problems that the popular version of the battle is all too happy to gloss over – most importantly (and surprisingly) the fact that our sources disagree on the number of Spartans.

The earliest surviving written account is that of Herodotus of Halicarnassus, who was actually born a few years before the battle, and lived in a time when its story was widely known. He tells us there were 300 full Spartan citizens (i.e. Spartiates) at Thermopylae, and treats this as the whole of the Spartan contingent.

However, other authors tell us the Spartans sent 1,000 men to the pass. We find this number for the first time in the works of the orator Isocrates, who lists a number of notable Spartan feats of heroism, and urges his listeners to remember “the thousand who went to meet the enemy at Thermopylai” (Archidamos 99-100). In the later account of Diodorus Siculus, Leonidas “announced that only a thousand were to join him for the campaign” (11.4.2); Diodorus later specifies that this force included 300 full citizens, with the other 700 implied to be perioikoi (“dwellers about”: the free but non-citizen inhabitants of Laconia).

In fact, this number of 1,000 Spartans, of which the famous 300 were only the Spartiate share, is already suggested by an epitaph cited by Herodotus (7.228.1). Eulogising the entire Peloponnesian part of the army, it says that “here once fought against three million / four thousand men from the Peloponnese.” But the numbers Herodotos gives us for the other contingents from this area don’t add up to 4,000 – unless we assume the Spartans sent 1,000 rather than 300 men. Even then, the numbers still don’t add up, but at least they come a lot closer. And could it be coincidence that the exiled Spartan king Demaratus tells Xerxes that the Spartans may march out with just 1,000 men to fight him (Hdt. 7.102.3)?

In short, we have good reason to believe that Herodotos deliberately suppressed the contribution of the Lacedaemonian perioikoi in the battle, writing the story as if they were never there. It is most likely that he did this in order to magnify the role of the Spartiates themselves, and to make more use of the number 300, which was charged with meaning by other heroic Spartan tales. There may, in fact, have been 1,000 Spartan troops at the pass, of which 300 were full citizens, and it was perhaps only the latter who stayed to fight to the death.

As for the Greek force as a whole, the ancient sources give its numbers as follows:

Modern accounts tend to give a total of about 7,000 troops, which relies on raw assumptions about the size of the Locrian levy, and which quietly accepts that there were indeed 1,000 Spartans, not 300. It’s important to add that none of these figures include even an estimate of the number of helots and other slaves present, even though Herodotus repeatedly states that they were there, and that some stayed to the end.

When the Greeks learned that the pass had been turned, most went home, considering the battle lost. But when it comes to those who opted to stay, again, totals vary widely among the ancient sources.

According to Herodotos (7.222), the Spartiates, Thespians and Thebans remained – a total force of about 1,400 men. Diodorus (11.9.2), however, claims only the Spartiates and the Thespians stayed behind, and states that Leonidas was left with just 500 men. Pausanias (10.20.2) has it that the Mycenaean contingent also decided to fight to the death, which would mean a total of 1080 on the final day. Justin (2.11.7-15) says only the Spartans remained, but gives their number as 600, presumably including as many perioikoi as full Spartan citizens.

It is impossible for us to say which number is the most credible. Herodotus’ very hostile account of the Thebans, who supposedly turned coat at the last second, shows that even his – relatively contemporary – account is already contaminated by propaganda; Plutarch spends some time dressing down Herodotus for this bit of slander (On the Malice of Herodotos 31). The only thing all sources agree on is that the 300 Spartiates weren’t the only ones to choose death.

In short, the Spartans didn’t just send 300 men to Thermopylae. By the lowest count, they sent an allied force of 4,000, but much larger totals are known. So while the claim itself isn’t necessarily true or false, it is misleading.