(2) Why nationalism?

[Geoffrey S]

I'd have thought patriot would apply more to pride in a country, with nationalist applying more to someone so fearful of his own nation's inadequacy that he sees other nations as a natural enemy trying to put down his own.

They allied all right, but it was not because of nationalism, there wasn't even any nation! They allied because they found out that it was a wise thing to do, kinda like England and France found out that it was a good idea to ally against Germany.

As I've said, I think you've been watching 300 too much

And it wasn't even a majority. A few citystates banded together, some joined the Persians of were conquered by them, and even more stayed well out of it. After that they all went back to killing each other, until someone else killed more of them and went off to Persia again. And in general, Greekness had nothing to do with it; such a word would have been meaningless in any case.