The Blood Service by Allen Ivers

Seventeen-year-old Aaron Havenes was sentenced to a lifetime of labor, serving the Empire till his dying day. But when conflict summons away the colonial defenders, his new home faces annihilation from an alien threat. A few Imperial volunteers defy their orders and stay behind to fight, but they need help–and they’re offering clemency to any convict that picks up a rifle to stand with them.

But Colonel Marcus Riley is a pragmatist: he has few good fighters and he can afford to sacrifice them. These convicts in uniform can be the losses he otherwise can’t afford to take.

Aaron is no fool. He didn’t even expect to live out the year–let alone become a folk hero, a pawn for political forces, or to draw an uncommon interest from his enemy. His war for survival is about to overturn an empire…