Warrior's Curse. The Warrior Chronicles, #5

Cort Addison is back on Earth to quell rising protests against his wars throughout the galactic arm. For the people of his home planet, exterminating an entire species over a kidnapping was the final straw. He finds himself facing xenophobic leaders on Earth, sabotage, and his own personal loss and betrayal, all while trying to find where thousands of humans are being abducted to. No one is prepared for the barbarity they find on the hidden planet at the other end of a natural wormhole, where Cort must battle an enemy more dangerous and ruthless than any he has encountered before.
Cort knows how to solve the problem: The same way he solves all of his problems, with brute force. But this time the cost of victory may transform both Cort and the entire Ares Federation forever.