The Art of Victory by Martha Keyes

Diana Donovan was raised aboard a naval ship on a steady diet of war strategy, so when her orphaned friend, Lucy, is denied a perfectly respectable love match by her controlling guardian, Diana sees an opportunity to put her battle knowledge to use in service of a deserving friend.
But Marmaduke Russell is not the middle-aged curmudgeon Diana is expecting. He is a maddeningly cool, unmarried gentleman who seems uniquely able to parry her tactics—and infuse her with even more determination to win. As battle ensues, though, it becomes clear that there may be more hearts in the balance than just Lucy’s.