User blog comment:Imperium Guy/Battle of Thermopylae: Alternate Outcome/@comment-32656-20120326140810

Well, I have to say: you both are wrong.

It is estimated that the Persians had 70,000 - 300,000 men in their army - the number I've seen most often is 200,000 or so. The Spartan-led Greeks are estimated to have had 5,000-11,000 in their force, with 7,000 or so usually being the quoted figure. The so-called "last stand" of the Spartans was more than just them, and in addition to the 300 of them, included 700 Thespians, 400 Thebans, and whatever helots - slave militia, for all purposes, but still good troops - survived of the thousand that the Spartans had brought with them in addition to their 300 - likely at least half of them remained at that point. The "300" bit is a myth. At the end of the battle, there was thus, at least, about 1.900 men there, and at the start of the battle, 7.000. And, those Thebans surrendered before the end of the battle, too.

Out of the 200,000 men Persian Army, it is estimated that 20,000 died in battle. And, of the Greeks, it is estimated that about 2,000 of them died in battle - most of which were the "last stand."

So, as for the question posed here....

Well, if the path had not been exposed to them, there's little doubt it would have been found eventually. Or, the Persians would have killed off the Greeks - make no mistake, it was not just Spartans - besides, likely after exhausting them. By switching out troops the Greeks could stave it off, but it would have happened eventually.

All told?

At most, before either they are killed off, or the Greek Navy is forced to retreat, forcing the army to retreat as well, the blocking force at the pass would manage to double its number of kills. So, 40,000, at most, dead Persians, from the battle. Of course, that's assuming that low supplies don't force a Persian retreat before then, which is also possible. But even in that event, still at least 20,000 dead. And, no amount of Persian dead would force them back, as an fyi.