A lot happens in Half a War, as one would expect from the conclusion to the trilogy. An alliance of three kingdoms, largely held together by Yarvi's manipulations, fights against the much more powerful High King and Ministry. The alliance is plagued by constant in-fighting, and nears defeat several times. The various ways in which the alliance is held together, and overcomes its enemies, makes for a consistently entertaining read.
Like the first two books, the viewpoint characters in Half a War are coming of age against the backdrop of the larger conflict. Koll is torn between becoming a minister, or following his heart. Skara has to learn very quickly how to lead her kingdom while helping to hold the alliance together. Raith has always seen himself as nothing but a warrior, but finds other aspects of life becoming more important - an illicit romance with Skara driving that change. The personal touch of these individual stories nicely offsets the great power struggle and army battles.
Like Half the World, this book also has plenty of appearances by existing characters. Yarvi continues to be the driving force behind the major conflict between nations, becoming even more of a Machiavellian manipulator. Thorn plays a fairly major part also, continuing in the warrior role that she earned in the earlier book. The kings and court of the countries in the alliance are familiar faces (other than Skara). I don't think that Half a War stands alone quite as well as Half the World did, with so much happening that has its roots in the earlier books. Probably not a good idea to pick it up before reading the rest of the trilogy.
Explicit subject matter does appear regularly in the trilogy. Graphic violence is common in all three books, though I don't feel like it was overdone. There are some sexual encounter scenes, and references to abortion. Several major characters spend time as slaves, and many more are slave-owners with no apparent regret. If any of that is a major problem for a reader, then this may not be the series for them. I didn't find any of it to be a problem, but rather felt that these things added to the depth of the world-building.
The history of the world becomes more important in this book, as some of the characters actually make a trip into the ruins of an ancient city. What they find there literally turns the course of the war for the alliance. There's little doubt that some kind of nuclear holocaust destroyed the ancient world, though Abercrombie never states anything explicitly. There's plenty of hints in what the characters encounter, so he doesn't have to. It's a bit of a stretch to believe how smoothly the trip into dead city goes, and how well-preserved they find their targets. But the author clearly wanted to focus more on the living world, not the ruins of the dead one, so I found it pretty easy to accept and move on.
The entire Shattered Sea trilogy was an enjoyable read, though it does deal in some explicit subject matter. For those who don't mind that, it's a very well-crafted story that I can certainly recommend.
