Exodus: Empires at War: Book 14: Rebellion.

Sean and the leaders of his fleets are determined to strike deeper into the enemy empire, liberating billions of slaves along the way, including their Maurid allies. The enemy threatens their own helpless subjects, knowing that humans will come their aid. They hope to destroy entire human fleets with each ambush. But the humans, who value loyalty both ways with the people they support, are not willing to let the helpless die, while also not putting their own necks on the chopping block. They have the people to carry out their plans and save those they have come to liberate. It is a risk, but sometimes those are necessary, and they have a plan. But plans don't always work out to the satisfaction of the planners, and failure here could lead to the Cacas changing the balance of power, and reversing the course of the war. However, the Ca'cadasan Empire faces another problem, as the Church of his people disputes his right to kill so many, and sets up a showdown that could bring down the Empire upon his head,