Use of Weapons is a science-fiction novel of The Culture by Iain M. Banks. It focuses on Special Circumstances and their extensive use of the Omniscient Morality License via the character of mercenary-extraordinaire Cheradenine Zakalwe, a man from outside of the Culture, but who does their dirty work.This is actually the first novel Iain Banks wrote, although it was published much later. Many consider it his masterpiece. As a side note, this article completely fails to do it justice, so be prepared for emotional gut punches by the dozen if you actually read it.
Use of Weapons provides examples of the following tropes:
Anachronic Order: It tells one story going forward with alternating chapters going in the opposite direction to provide flashbacks to Zakalwe's past. This is so confusing with respect to the first and last chapters, that fans disagree on whether they detail past or future events. One online reviewer jokingly referenced the novel as The Melancholy of Cheradenine Zakalwe.
The Atoner: Zakalwe tries and fails to be one of these. Although the ending of Surface Detail suggests that he is still trying and has, to an extent, succeeded.
Badass Bookworm: Sma is a poet and patron-of-the-arts as well as an extremely competent Special Circumstances operative.
Bait the Dog: For a lot of the book, it's easy to think of Zakalwe as a really cool and Badass secret agent and to think that his handlers from the Culture are off-base when they refer to him as a dangerous psycho. Certain events in his past change that impression.
Becoming the Mask: Elethiomel borders on this in a twisted way; part of him really has come to believe that he is Cheradenine.
Cain and Abel: Cheradenine and Elethiomel Zakalwe are an interesting take on this, as the flashbacks present Cheradenine as something of the Cain, being a jerk to his adopted brother Elethiomel then it turns out that the Cheradenine of the novel is actually Elethiomel and he did a Moral Event Horizon crossing event that made him the Cain figure.
Coitus Uninterruptus: Implied at one point when Zakalwe calls Sma up to report on his mission. She takes his call, but seems... preoccupied.
Combat Pragmatist: Done on both a strategic and tactical level by Zakalwe and Elethiomel, especially in the flashbacks (because the latter has assumed the former's identity). It's not called Use of Weapons just because of the guns.
Continuity Nod: Before setting out to recover Zakalwe, Sma tells Skaffen-Amtiskaw to "send a stalling letter to that Petrain guy." The novella State of the Art is introduced as a later communication from Sma to Petrain recounting her time on Earth.
Genius Bruiser: Zakalwe doesn't look particularly burly, but he definitely fits the trope. He's an insanely competent (and insane) soldier, but he's also clearly a genius. Despite coming from outside the Culture, he figured out how to copy several Culture technologies, and can out-think Special Circumstances.
History Repeats: Zakalwe is painfully aware that he has a bad habit of repeatedly making the same mistake of over-entrenching in an unwinnable situation and then not withdrawing when he should. He is otherwise depicted as a brilliant military mind, capable of turning the tide in a war despite overwhelming odds. At the end of the book, he accidentally wins an "unwinnable" war he joined the "wrong" side of, at ruinous cost of life and treasure, and when informed, he goes into a Heroic BSOD.
It Works Better with Bullets: A king wakes up with an assassin in his bedroom. While he's being taunted, the king pulls a gun and tries to fire. The assassin off-handedly shows him the bullets and says, "It works better with these."
Losing Your Head: Special Circumstances operative Cheradenine Zakalwe crash-lands on a primitive planet and is sacrificed by the natives through decapitation. Fortunately his colleagues zoom in just in time to snatch back his head, but not before he's had a horrified moment to realise exactly what just happened. Later Zakalwe is in hospital waiting for a new body to be grown (they gave him the choice of remaining unconscious but he'd rather watch television) when the artificially-intelligent drone Skaffen-Amtiskaw (who doesn't like Zakalwe much, and has a twisted sense of humor) sends him a present. A hat.
Monster Clown: Zakalwe commits an assassination dressed as a jester, and the victim sees him as such, but it is suggested that Zakalwe's strikingly colourful outfit wouldn't look at all out of place in a Culture gathering.
Mr. Vice Guy: Sma has, shall we say, a healthy attitude towards sex.
Skaffen-Amtiskaw: Micro Armaments System, Rifle. It's... oh, look, Zakalwe; it has ten separate weapon systems, not including the semi-sentient guard facility, the reactive shield components, the IFF-set quick-reaction swing-packs or the AG unit, and before you ask, the controls are all on the wrong side because that's the left-hand bias version, and the balance — like the weight and the independently variable inertia — are fully adjustable. It also takes about half a year's training just to learn how to use it safely, let alone competently, so you can't have one.
Twist Ending: The two main approaches to the ending is one story working forwards towards the events and a second story working backwards to their meaning. Keeping it a surprising twist under these circumstances is an achievement. The twist is that Cheradenine of the novel is actually Elethiomel from the scenes set in childhood. It's subtly alluded to shortly beforehand: "The doctors were still struggling to save [Charadnine's] life when the soldiers trapped in the battleship made a break for it. It was a good fight and they almost succeeded." The doctors "almost succeeded", that is. Apparently, Elethiomel's forces did break out of the siege.