Mahan's argument for fighting over principle

Mahan treats the prevention of war as a statesman's achievement. He nevertheless argues that disputes can involve convictions of right that neither side will surrender. His American Civil War example goes further, claiming that retreat from sincerely held principle would have been worse than fighting. He distinguishes negotiable material interests from convictions whose abandonment he regards as a loss of character. This is his moral justification of war, including his allowance that a conviction can be honestly mistaken.