The Hot Gate — John Ringo

The fight to free the Earth from alien domination began in Live Free or Die, and continued in Citadel. Now Tyler Vernon, and his troops aboard the gigantic battle station Troy, face a desperate battle with the forces of galactic tyranny. And the very survival of the Earth and its people is not all that is at stake. The galaxy itself must choose to live free or die -- and if the tyrants win this battle, darkness will fall across the galaxy for millennia to come.