I never saw anything but rocks in either the comic or the movie, but just for the sake of the scenario, lets say that Spartan lands are highly fertile. Ragnar who has had his fill of England and finds Paris too hard to chew, wants to expand and settle in proto Greece.
The same as when Xerxes tried to roll him, Leonidas finds himself stepping to the plate without initial support from the other states.
Ragnar is joined by;
Rollo
Björn Ironside
Floke
Lagertha / Earl Ingstad
300 assorted vikings raiders
Leonidas gets his 299 'bodyguards' and decides to take a stroll by the cliffs after recieving word of the approaching long boats.
A) they face off straight away
B) 1 week of mutual prep