This is an interesting theory about why we fight wars. Quite a long and dense read but worth persevering with. My summary below does not follow the same order as the book.
Humans, gorillas, chimps, bonobos and humans are all sociable and territorial species that evolved from a remote common ancestor. Proto-Pan split into Gorillas and the remaining apes. Then humans split from chimps and bonobos. Finally, location was responsible for the split between chimps and bonobos.
Chimps and bonobos both live in Africa and both eat mostly fruit and seeds (and when they can catch them, monkeys). Bonobos live south of the Congo river and expanded their diet to include eating leaves and shoots, which are more plentiful, developing teeth with long, shearing edges to do this. Chimps live north of the Congo river and share habitat and compete for food sources with gorillas. They did not develop teeth suited to leaves and shoots. The difference in diet influenced strategies for finding food, and social structure.
Bonobos travel in large,stable groups of around 16 individuals whereas chimps split into small, often male single-sex groups of 2 to 8 and females forage individually. Male chimps often rape isolated females, while male bonobos court females. Chimps fight and kill to defend or increase territory, while bonobo groups share food, grooming and sexual partners with other groups: sex acts can be male/female, male/male or female/female.
Early humans it seems went for the middle way. Like bonobos they shifted from fighting for mates to courting them, and pair bonding. But like chimps, they continued to use violence as well. In both chimps and humans, violence is largely committed by young males. Unlike either, humans have large brains and culture. Cultural evolution is faster than biological evolution.
Archaeological finds suggest that Neanderthals were very violent. Skulls show traces of healed non-fatal stabbings. Head and neck trauma suggest fighting (some prey injuries but likely fellow Neanderthals) and there is evidence of occasional cannibalism. Statistical research indicates that Stone Age societies were 10 to 20 times more violent than medieval Europe, and 300-600 times as bad at mid-20th century levels.
Tribes compete for territory and resources. As they get larger, they need 'government' or administration - taxation, armed forces, and 'policing' of violence. If they get too large (empires), they can fail and are conquered by others. Governments pursue what they perceive as their best interests, not what philosophers propose.
Empires have good and bad emperors, but they and the ruling elites pursued enlightened self-interest, and in doing this also left most people better off. They became stationary bandits, who avoid stealing everything and leaving, but stay around, promote prosperity, so there is more for future rulers to take. It is the bureaucrats, lawyers and hangers on who make it work.
Territory increases to a point where the rulers need to delegate tasks to trusted allies, but eventually get too large and collapse due to epidemic disease and invaders with better weaponry. Over time new empires rise.
Up to the 20th century, the biggest killer in war was always disease. For thousands of years each of the early civilizations had evolved its own unique disease pool. Between a quarter and a third of all babies died within the first year and few adults survived past 50. As empires grew, migrants moved taking disease with them. And war, with thousands of men in small spaces, badly fed in filthy conditions where exotic viruses thrived was even more efficient at this. In AD 161 a puzzling new disease (possibly smallpox) killed one-third of men in a few weeks and spread around the Roman empire.
As empires collapse, there are fewer people and rulers struggle to get soldiers for armies and taxes to pay them. Add in climate change (colder and drier) and crops are poorer. Frontiers collapse and great migrations spread disease even faster. Soldiers turn into bandits, generals into warlords.
Kingdoms unable to squeeze money for armies from the aristocracy, cut deals with them instead - feudalism. As generations passed, lords often ended up with multiple, and sometimes conflicting, obligations to several kings.
War is often a time of innovation, and societies and cultures change. As empires collapse, it takes time for new ones to rise. When empires get large enough, they can act as globocops but this requires great resources. From the Roman empire to the British empire, they are caught with spiralling costs.
In Western Europe, after WW2, politicians became alarmed that a reunited Germany would 'rerun the historical script'. However, far from turning into a rival to the globocop, Western Europe has almost entirely renounced force as a policy tool and in so doing, building a bigger, safer, richer society without being forced to do so. The USA has effectively become the globocop but needs allies to help spread the load.
Nuclear weapons are expensive and complex. The 'prisoners dilemma' - who can you trust?
Overall, violence has declined dramatically since the Stone Age. Perhaps only when societies are so pacified that violent death falls below 2%, are women sufficiently empowered to challenge male aggression and societies become more feminised. Today, homicide is in general retreat. In 2004, 1 human in every 13,000 was murdered; in 2010 the figure is 1 in 14,500.
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