"The Space Barbarians" is a science-fiction novel by Mack Reynolds, published in 1969 as one-half of an Ace Double, with the other half being The Eyes of Bolsk by Robert Lory. "The Space Barbarians" is apparently not set in the "United Planets" universe of The Rival Rigelians and Code Duello, although thematically it is close to both: a "lost colony" which has resorted to a an earlier period in earth's history is recontacted by a "galactic federation" of some sort, and is reintegrated into modern customs. More or less. This book is also the longest Ace Double I have read, at 155 pages, which is good, because it covers a lot of ground.
The main antagonist of the story is "John of the Hawks", who at the start of the book is only 17 years old, but over the course of the story becomes a war chief. As we are efficiently informed of pretty early in the story, the story takes place on a world where colonists from Scotland crashed, losing much of their technology and history (as a tagline on the cover informs us, only four books survived the crash), and reverting to a clan based system based on ceremonial combat and cattle raiding, and with strictly enforced cultural "banns" on many forms of technology. Despite constant small scale warfare, these rules prohibit too much destruction. For example, repeating rifles are forbidden, and warriors attempt to "count coup" on each other. Into this not-exactly-peaceful world drops a spaceship from "beyond". The Caledonians are not too surprised at this, since they at least theoretically knew their history. However, they consider the new arrivals to be interlopers who don't understand culture. Unfortunately for them, their planet seems to be very rich in platinum, to the point where they consider it less useful than iron. The new arrivals hope to mine the planet, and set out for a decades-long quest to subvert the world so they can mine it. This is paralleled by John's rise as a warchief, and his attempts to keep his planet free.
This book was published in 1969, which means that the ongoing Vietnam War would have been an obvious backstory. So, yes, this book reads as "What if the Vietnam War was in space but also the Vietnamese were medieval Scotsman?", and it is not as ludicrous as it sounds. For one thing, the "medieval" setting allows lots of sword fighting, which is always popular in Ace Doubles. Perhaps surprisingly, given the short length of the book (but long for an Ace Double) and many action scenes, the book manages to make some involved points about social development and the deficiencies of all systems. The feudal system is not always glorified. It is good for action scenes, but the story does point out that despite the rough "democracy" of the clans, it is a system that doesn't allow change. The "capitalist" system is also criticized, especially with the exploitation and manipulation of tribal groups. There are also parts of the story that are outside of the metaphor of colonial conflicts of the time, such as the fact that most of humanity outside of Caledonia worships Krishna, and uses a psychedelic drug called Soma---which is also used to pacify people. The book takes some twists and turns, both ideologically and storywise, before reaching its conclusion.
This book is an example of how science-fiction can be used to describe social conflict and social development. Parts of it were a bit too preachy, some of the action scenes were predictable, there was a superfluous romantic subplot, but in general I thought that it was, like most of Mack Reynolds' books, entertaining and insightful.