By S. M. Stirling:The Tears of the Sun: A Novel of the Change

Rudi Mackenzie has traveled from the land where the sun sets to the land where it rises and back. He has found his weapon—the Sword crafted for him before he was born. He has made friends from among his enemies and found enemies where he expected friends. He has won the heart and hand of the woman he has loved his entire life.  Now Rudi is Artos, the High King of Montival, and his final destiny awaits him. He must face and defeat the forces of the Church Universal and Triumphant. Everything in the present, everything in the future, depends on the outcome of the conflict. And like his father before him, Rudi knows that in winning the war he might well lose his life...