The End of All Things (The Old Man's War Series)

Humans expanded into space, discovering multiple alien species. But these were enemies not allies, their forces were bent on our destruction. Thus was the Colonial Union formed, to protect us from a hostile universe. And Earth's excess population became its colonists and soldiers. It was a good arrangement - for the Colonial Union. Then the Earth said no more.
Now the Colonial Union is living on borrowed time before its defence forces are fatally depleted. Then struggling human colonies will be vulnerable, easy prey for alien species eager for humanity's ruin. And there's another problem. A group lurks in the darkness of space, playing human and alien factions against one another. They'll even target their own kind and their reasons are unfathomable.
In this collapsing universe, Lieutenant Harry Wilson and Colonial Union diplomats race to discover who's behind these attacks. They must also seek peace with an angry Earth, and keep humanity's union intact. The alternatives are oblivion, extinction - and the end of all things.