The Once and Future King Book 2, Chapter 3
On a hunt with Arthur, Kay asks Merlyn who Queen Morgause is, and why her husband, King Lot, is fighting Arthur. Merlyn tells them simply that Lot fights Arthur because Arthur's ancestors have been conquering Lot's ancestors for thousands of years, and because since Uther raped Morgause's mother, Morgause hates Arthur as Uther's son. Merlyn doesn't believe that any of these are good reasons for a war.