Fans have been trying to figure out if Battle of Artemisia will be a sequel or a prequel to 300. The answer is probably "neither."
300: Battle of Artemisia (originally titled Xerxes) covers the story of another battle which occurred simultaneously with the one depicted in the first film - which had a great impact on the events we already know.
There will likely be naval combat involved since, in real history, Artemisia I of Caria was one of Xerxes I of Persia’s allies, and her only military role was as a naval commander in the Battle of Salamis, which Persia lost after Xerxes failed to take Artemisia’s strategic advice.
That loss became a turning point for the conquest of Greece by Xerxes. When he was unable to overcome the Greek navy at Salamis, he stopped expanding into Greek territory. Of course, in reality, the battles probably did not truly occur simultaneously, though we only really know they were both held in the final year of Xerxes campaign against Greece.
"We start shooting in July. They’re working on the ... last draft of the script. I think they’re calling it a ‘spin-off’ because it’s not exactly a prequel, it’s not a sequel," co-star Rodrigo Santoro told Collider.
"It takes place in the same time, you start a little earlier but then you see that it’s like a different point of view, a different camera, a different angle. While that battle was happening, the one that you saw in 300, there was another battle happening next to it and you will have a chance to see all of that happening and how they interact."
He went on to discuss more about his own role, and how important the character of Xerxes, dramatically presented in the Frank Miller novel and the first film as a god-king will be portrayed in Battle of Artemisia.
"I think they will try to do a little backstory on Xerxes and they will explain how he became ‘the God King,’ which I’m excited about," he said. "I don’t know what it’s going to be. I’ve never played a character twice. It’s interesting. I think the challenging thing for me now will be to try to bring something new for the fans, for the audience and for myself just to make it interesting to play it again."
300: Battle of Artemisia, which also stars Eva Green in the title role, begins primary photography this summer and is slated to hit theaters on August 2nd, 2013.