It's an original story, so it's a little hard to explain. Earth isn't involved, it's in a ficticious galaxy. Humans once ruled a quarter of a galaxy, then they fell from power and are on the verge of annihilation by various nonhuman races. They are confined to a region near the galactic core known as the Human Sphere which, in its stupidity, decides to convert to pacifism despite the threat from the nonhumans.
High Admiral Legan, supreme commander of the dominate nation within the Human Sphere called the Kataran Republic, decides to steal his military away from the Senate before they can enact their pacifistic legislation and disband it. He takes his fleet out into wild space along with a number of civilians and begins to build a tiny rogue empire in the hopes of one day building up a force that can defend humanity against its own stupidity and the more advanced nonhumans.
Legacy takes place a few years after Legan left the Sphere behind. The official synopsis follows:
After setting up his fledgling empire in the unknown regions of space, High Admiral Legan goes on a quest to locate Crestanya, the armory world, in the hopes of acquiring the advanced technology that he needs to defend humanity against another nonhuman invasion.
Meanwhile, the Cres Empire has fallen and the Emperor sends Mysonanda back to the humans with a plea for help, but when she arrives she finds that Legan has already left…with no way for her or anyone else to contact him.
Faced with a timetable they can’t ignore, Legan’s subordinates must make the hard choices in his absence. Do they protect their anonymity and turn away the Cres, or do they risk Legan’s entire operation by aiding the hunted nonhumans in exchange for access to their superior technology?