A Question of Faith (Castle Federation, #6.5)

Two star nations struggle for primacy
A protectorate of both left defenceless
A call for help from any who can hear

The Castle Federation and the Coraline Imperium have built pocket empires on the fringe of human space. Tiny by the standards of the behemoth Commonwealth born of Terra, they remain unchallenged in their own sectors—but their fleets watch the border between them with careful eyes.
When the Boudicca System’s only battleship is ambushed and destroyed by forces unknown, their leader calls for help from the Federation and Imperium alike. Admiral Darius Moonblood of the Castle Federation takes a fleet to Boudicca to answer that call, believing he is walking into an Imperial trap.
It will fall to Admiral Moonblood to either start the war Castle and Coraline have feared for years or find a different answer to the conflict looming in Boudicca.