"go tell the spartans, thou that passest by, that here obedient to their words we lie." - herodotus, trans. by godley. epitaph for the battle of thermopylae.
400's bc. persia is a giant empire that intends to annex (what is now) greece. mostly works but athens and sparta resist. internal strife causes the persians to pause for a bit, but they return with a super force to crush the rebel cities. two and a half million according to herodotus (which is definitely off by at least an order of magnitude). greeks engage the persians on a narrow pass killing a lot of persians. eventually persians found an alternate route and threatened to encircle the greeks. 300 spartans (and a bunch of others) remained. one of the most famous battles of the ancient world -- freedom loving city-states against the evil empire of the persian king of kings, xerxes.
to the persians, this was probably just another border skirmish. but they aren't the side writing the book. (and persians do enter a decline that makes them less of a threat to greece moving forward.)