Tactics of Mistake (1970) is the story of Cletus Grahame, Donal's great-great grandfather. (At some point after settling on the Dorsai, Cletus returned his surname to its original Celtic spelling.) In a reprise (or prefiguring) of the beginning of Dorsai!, we first meet Cletus on board a ship where he meets two people who turn out to be quite important. Dow DeCastries is a Secretary of the Eastern Coalition of Earth. Melissa Khan is a beautiful young girl, the daughter of a Dorsai officer. She is, (of course) both attracted to Cletus and repulsed by him. Cletus, as an officer for the Western Alliance of Earth, is in principle DeCastries's enemy. The two blocs aren't at war officially, but they tend to fight proxy wars on less-developed planets. (The resemblance to the Cold War is clearly intended.) Cletus is on his way to fight in the current one, on the Exotic planet of Kultis, where a guerilla insurgency supported by the Coalition threatens the legal government, which is supported by the Alliance. (The resemblance to Vietnam is obvious.)
The plot of Tactics of Mistake is the same as that of Dorsai!. Cletus goes from success to success, pursuing his ultimate goal, which is the Dorsai as an world of professional soldiers whose skill defends the independence of the Younger Worlds from Earth. To this end, he continually antagonizes DeCastries, whom he recognizes from the first as a pivotal figure. At the climax, DeCastries brings a combined Alliance-Coalition army to the Dorsai to try to capture Cletus. When Cletus leads the Dorsai to defeat this army and instead captures DeCastries, his victory is complete.
Even though many of the Childe books could be called milSF, Tactics of Mistake is the one that most deserves that name. The battles that Cletus fights, particularly his first engagement on Kultis, are described in great detail. Cletus is a brilliant tactician, with a knack for guessing what the other side will be doing and inventing countermeasures. The Kultis battle is reminiscent of an Admiral Naismith victory, but more realistic and told with much less humor.
In many ways, Tactics of Mistake is a retelling of Dorsai! starring characters who are less extraordinary. Cletus is a genius, but not an otherworldly one. Rather than relying on a talent no one else can imagine, much less duplicate, he's writing a series of books to explain his ideas on strategy and tactics, and throughout the book he gathers and teaches disciples. Nor is Cletus odd the way Donal is; he's actually quite likeable and charming. (He's also very tall, like most of his descendents.) Dow DeCastries is an ambitious politician, not a driven monster. He sees defeating Cletus as his ticket to supreme power, but he doesn't seem to hate Cletus. We see his pet savant, Pater Ten, only briefly; he's nasty and waspish, but doesn't seem capable of creating the sort of havoc ArDell Montor caused. There's no parallel to William's torture-murder of Mor. Melissa is a strong, spirited woman, but there's no suggestion that she's more than that. (Though Dorsai! never shows us anything extraordinary about Anea, it tells us over and over that she's one of the great successes of the Exotic breeding program.)
Cletus stars the book with a limp; he has a partially prosthetic knee from a battle wound. His body rejects all attempts at a transplant. He's eventually cured by a miraculous form of regeneration aided by Exotic psychology. The parallel to Paul Formain's desire for the Chantry Guild to grow him a new arm is obvious. The oddest thing in the book, to me, is Cletus's treatment of Melissa Khan. She's obviously attracted to him, and her father, whom she worships, like and approves of Cletus too. Yet instead of wooing her the normal way, he forces her to marry him, and tells her that his only interest is keeping her father near as a valued subordinate; once the crisis with DeCastries is over, he'll consent to an annulment. At the end of the book, she tells him that she married him only because she knew all along that he loved her. Which is apparently true; we know that their marriage was successful and led to many generations of Graemes. This is a huge and entirely gratuitous bit of cruelty on Cletus's part. Again, it's a bit reminiscent of Miles's inability in A Civil Campaign to treat courtship as something different from warfare. But Miles is appropriately humbled for that before he gets his girl; Cletus isn't. The whole subplot is distasteful, and serves no obvious purpose.