Is there no other way to recruit Kurohime? It seems odd that you'd build a good standing with Dorgania for a quest, help them with their repairs, sleep with their princess, then randomly declare war. Well OK, I'm mainly more concerned about the relationship with the princess than Dorgania itself. If I could conquer them without upsetting/loosing anyone in the harem I'd put them to the sword without a second thought. The princess is a well written character and has a heartwarming storyline, it'd be a shame to throw it all away...