This book was a 2.5/5 for me for most of the book, and declined towards the end. Spoilers follow.
"The Year the Horses Came" is a novel inspired by a theory about human prehistory. According to this theory, in the distant past, all or most human societies were matriarchal, and these societies were violently overthrown by patriarchal societies in the more recent past (~5000 years ago). This theory has been on the decline, but it is still interesting to look back on older stories and see what kind of exotic cultures authors could imagine. As I began this book, I read it as a kind of fantasy story, and enjoyed it at first.
The story follows Marrah, a villager in a matriarchal society in Western Europe. A foreigner named Stavan washes up on her shores. His strange appearance and customs seem exotic and shocking to the local villagers. He talks about strange animals that could be ridden (horses), nomadic life, warrior customs, monogamy, the subjugation of women, and various other topics that are incomprehensible and shocking to the local villagers. After a village elder starts to see a connection between the things he says and a prophecy about an invasion from hostile invaders, Marrah and a now-sympathetic Stavan (and Marrah's younger brother) are sent on a quest to carry this information eastward to the other peaceful villages that are under the threat of invasion. Much of the book is dedicated to the journey of these three characters traveling prehistoric Europe and experiencing the local customs.
Mary Mackey does not depict matriarchies and patriarchies in a remotely mature or realistic way. Matriarchies are completely peaceful and sympathetic, and patriarchies are completely violent and loathsome. For anyone who has read anything about the subject, it's clear who the two sides of the conflict are meant to represent: Pre-Indo-European Europeans in the case of Marrah's society, and Indo-European invaders for Stavan's tribes. Marrah practices a kind of polytheism which has been carefully stripped of anything modern people would find shocking or unethical. Stavan's people practice a kind of polytheism that has been stripped of anything most people would recognize as humane or sensitive. They enjoy relentlessly killing and maiming, including animal cruelty and violence against women. Does it not occur to Mackey that a society that lives their entire lifestyle in a symbiotic relationship with a particular animal (horses) might actually care for animals? Other contrasts also make no sense. For example, the narrator emphasizes multiple times that Marrah's culture values the elderly. Is Mackey not aware of the countless patriarchal societies that practice ancestor worship, or have images of elderly wise men as cultural touchstones? The narrator also emphasizes several times that Marrah's culture sees nature as a living thing, while Stavan's culture does not ("...and the Earth was a dead thing to them..."). But what does this actually mean, if it is anything other than emotionally charged rhetoric? Stavan's people also worship a pantheon of deities focused around forces of nature. They also live in nature and have a close relationship with animals--closer, in fact, than Marrah's, given that they spend much of their lives on horseback. So in what sense is it true that Marrah's people see nature as living and Stavan's people see nature as dead? One has to conclude that this is just emotionally charged rhetoric and nothing else. While these are all unrealistic portrayals of cultural differences, there is one detail in particular that is genuinely libelous: Towards the end of the book, Stavan's tribe forces Marrah's brother to undergo a circumcision, after accepting him as one of their own. All of the historical evidence available on circumcision indicates that this is not an Indo-European practice. In fact, patriarchal, polytheistic Europeans (the descendants of the "patriarchal revolution" people like Mackey believe in) found circumcision disgusting and criminalized the practice. I could list other features that are out of place or unrealistic, but it is easier to summarize Mackey's depictions of patriarchal societies as being based on shock value rather than anything substantial and historical.
Despite the unrealistic premise, a story could still be compelling on its own literary merits. This book fails at that as well. The shallow, idealized society Marrah comes from robs her and the other characters of any depth: All of the young villagers are friendly and wildly enthusiastic about everything they do, and all of the old villagers are patient and wise. Stavan seems like a complex and interesting character at first, but devolves into a plot device to communicate how miserable and worthless his own culture is. The dialogue towards the end of the book, which includes Stavan "admitting" his own people are miserable, brutal, and even bad sexual partners, is cringe-inducing and juvenile.
To be fair, there are a few things I did enjoy about the book, which is why I was originally going to give the book a middling rating. Mackey did depict elaborate customs, rituals and lifestyle features for Marrah's village as well as the other villages she visited. The prose is good, and the dialogue is believable earlier in the story. As a result, immersion is good in the first two-thirds of the story, and I enjoyed learning more about Marrah's lifestyle and the village. I was disappointed by what the story ended up becoming.