User blog comment:Imperium Guy/Battle of Thermopylae: Alternate Outcome/@comment-32656-20120326140810/@comment-32656-20120326214843

....That is ''not' what I said.

With an army of that size, it is very hard to keep it in the field. If they can be held there for a couple of weeks, the Persians will likely have to leave. Really, most of the Persian army died from this when retreating. It cannot be ignored.

If the path is not pointed out to them, it's something like 65-35 odds that they get forced back, and it's more likely to be from the fleet having to pull back than the army needing to do so.