From the announcement:
Greenville was just a skirmish – now the real war for Earth begins! Take your Mars Attacks games to the next level with open scenarios, larger maps and free selection of troops for your army of choice. Will you take the defenders of humanity, massing troops and trucks led by mighty heroes? Or the numberless hordes of the Martian Empire, led by the most dastardly and deadly warlords the galaxy has ever seen? Either way, it’s time to go big or go home!
Mars Attacks: World War and a new range of Mars Attacks Army Sets are now available to pre-order from all good hobby retailers and the Mantic Games Webstore. They will begin shipping in May!