Exteel actually has a back story, and you can find it over at GameZone.
In this story, we learn about the struggle between four colonies on the recently-inhabited planet of Natha. Each of these colonies has their own ideals and their own reasons for warring against the others. Some are refugees from Mars, some from Saturn and some from Jupiter, but they're all at war with each other. The reason they were once unified has crumbled away, and now they must continue their skirmishes in their metal behemoths until a resolution is found.