Tir Tanagiri, book 2: The King's Name
Jo Walton2001

The warrior Sulien ap Gwien and her lord King Urdo have finally united the land of Tir Tanagiri into a kingdom ruled by justice under a single code of law. But where many see a hopeful future for the land, others believe they sense the seeds of a new tyranny. Soon Tir Tanagiri faces the blight of civil war, and Sulien ap Gwien must take up arms against former comrades and loved ones, fighting harder and harder to hold on to Urdo’s shining dream.