Redeemer

Battle to save a doomed planet
When the Concordian colonists finally reached Earth after years lost in space they expected a short visit. All they needed was bio-material to reseed their world.
What they hadn't anticipated was the arrival of the Scythians.
They learn the deadly aliens attacked the planet once before, devastating the burgeoning civilization. Now they're here to stay.
Earth is weak. Humanity has barely begun to climb back to its previous level of development. The Concordians face a terrible choice: stick around and help in the effort to repel the invasion, or return home and save Concordia.
Each option carries its own risks, but with or without the Concordians' help, Earth seems doomed.
Redeemer is book eight in the epic space colonization adventure, Space Colony One.