There are at least three books about the decline of violence in human societies that should be read before this one: Steven Pinker's "Better Angels of our Nature", Joshua Goldstein's "Winning the War on War", and Jared Diamond's "The World Until Yesterday" (the weakest of the three). They lay to rest the myth that primitive societies were idyllic edens: they were brutal anarchic places where murder was an ordinary (not extraordinary) event.
Morris' book is a lengthy demonstration of a claim that initially appears senseless: in the long run, war has made the people of the world safer and richer mainly because it has created better organized societies that have reduced the chances of violent death. Archeologists estimate that, during the Stone Age, as many as 20% of people were murdered by other people. By comparison, no more than 200 million people died in all the wars of the 20th century, and that's not even 2% of the world's population in that century. The winners of wars typically build stronger governments and bigger countries, and their first goal is to suppress violence. The Romans, the Mongols and the Brits used brutal means to stamp out violence within the large empires that they created, but the truth is that many regions of those empires experienced more violence before being conquered and annexed. These larger and peaceful societies created by war also made their citizens richer precisely because the outcome of war is a stronger government and a more peaceful society in which business can prosper and lasting infrastructure can be built. It may sound odd that the destruction of wars leads to wealth, but that's the historical record: after endless wars all over the world, we are richer, not poorer, than our ancestors.
Pinker and Goldstein attribute the decline of violence to other factors. Morris focuses on war: war created the state, and the state created peace.
Morris then makes another claim that is a bit more controversial: that war is the only way that humans discovered so far to create more and more peaceful societies. Plenty of international treaties (and multinational organizations such as the United Nations) seem to have achieved what war achieved.
Morris' book seems to prove that Hobbes was right: human nature is evil and only strong government can create a better society.
Denmark stable peaceful and prosperous Victor Hanson's thesis was that ancient Grece invented the warfare practiced by Europeans for the last 2,500 years. Rome perfected it and since then all European powers copied that model. But Morris shows that Asia was no less successful at warfare.
Morris starts the story a bit later, in the age of the empires, about two thousand years ago, when several large, peaceful and rich empires formed: the Han in China, the Mauryan in Indian, and the Roman in Europe. These empires all happened to be located roughly at the same latitudes, which Morris calls the "lucky latitudes". These are also the latitudes where farming began (in Mesopotamia, the Yellow River valley, the Nile valley and the Indus valley). Morris' thesis is simple: farming made war worth it. Farming created the accumulation of wealth that justified going to war for (either to defend it or to loot it). Jared Diamond in "Guns, Germs and Steel" (1997) made the point that the "lucky latitudes" had the highest number of potentially domesticable plants and animals. That's why farming began at these latitudes. Farming in turn created densely populated regions. This is the phenomenon that Robert Carneiro called "circumscription" (1970) and Michael Mann called "caging" (1986). What followed was natural selection of organized societies: the better organized ones won over the others and annexed them. Caging forced people to create more efficient societies, whether they wanted it or not. That was the beginning of organized warfare, not just sporadic violence and looting. The devastation of warfare greatly increased with the invention of the cavalry. The horse was domesticated about 4,000 years BC in Ukraine and by 3,300 BC it was used to pull wagons across the steppes; and the chariot, that can carry a warrior, was invented about 2,000 years BC in Kazakhstan. The chariot was expensive so only rich kingdoms made large-scale use of it. It helped the rich empires get bigger, stronger and wealthier. But then the bronze sword was invented in abour 1,450 BC in the Alpine region between northern Italy and Austria. Warriors armed with bronze swords devastated the rich empires. In about 1,050 Cyprus invented iron weapons, which further increased the ferocity of warfare; iron weapons were cheap to make and as lethal as bronze ones. It became feasible to create large armies of foot soldiers armed with iron weapons (no need for chariots anymore). In about 900 BC the horse warrior came out of the steppes: one thousand years of selective breeding had created horses that were war machines, big and fast. Assyria pioneered the new kind of warfare that relied on state-of-the-art foot soldiers, a lot of them. Assyria's king Tiglath-Pileser III revolutionized warfare by drafting peasants into the army in return for land ownership: the state now had a reliable large army. Morris finds a similar pattern of warfare in China, just delayed by a few centuries because iron didn't spread as quickly in China, and also in India, delayed by a few more centuries. That's how the world got to the age of the Han, Mauryan and Roman empires.
All these empires eventually came to face the horsemen of the steppes (Mongols, Huns, Scythians, etc), the very horsemen from which, a thousand years earlier, they had learned the secret of the war horse. The empires had a simple choice: build a large cavalry or be overrun by the "barbarians" of the steppes. Parthia and Han China did so quickly, whereas Rome was late in doing so, and India didn't do in time (and was therefore overrun by the Kushan).
There is however a diminishing return to conquest: there is a limit beyond which the empire has more to lose than to gain from further expansion. That's precisely what happened to the Roman and British empires. The farther the armies stretch, the higher the costs and the lower the benefits. The triggering event for the Romans and others was the plague. But Morris summarizes the messy centuries that followed (the "dark ages" of Europe) as simply the consequence of war becoming unproductive. The story is way too complicated for us to accept Morris' simplistic explanation. For example, an empire continued to thrive in Iran (the Sassanids) while Europe, China and India were descending into chaos.
Anyway, a new age of empires began with the spread of gunpowder, and the European countries were particularly good at improving firearms. Morris spends 200 pages retelling the history of Europe (with a few detours into China and India) all the way to the Cold War. During the Cold War the world was on the brink of complete destruction. However, today we are richer and more peaceful than ever.
After the summary of human history, Morris' book delves into biology and archeology. Chimps are very similar to humans when it comes to violence, especially the violence of gangs of young males, but our histories diverged dramatically more than a million years ago, and Morris thinks that this is largely due to the male-female bonding (the invention of the family): human males hunt and the woman gathers and cooks, thereby creating a system in which the man has to court the woman and has to defend her against other men; whereas chimp males fight each other for the right to rape the female and nobody defends her. The human strategy prevailed and humans became by far the most successful ape of all. Humans evolved culture, and culture turns killers into conquerors: where chimps simply kill other chimps, humans build empires. And empires (while they last) yield peace and prosperity. Culture allows people to organize themselves into larger and larger societies, into more and more powerful states. The more powerful the state, the higher the cost of violence against it. Hence peace.
I am not sure that Morris' thesis follows logically from the condensed history of human civilization contained in those 200 pages, nor from his analysis of the behavior of chimps and bonobos and early hominids, but it certainly makes for an intriguing concept.
Morris ends with a lengthy discussion of the world in 2013 (the year in which he wrote the book). Of course most of the discussion pivots on the role of the USA as the world's cop. Like all global cops the USA has indirectly created its own rivals (notably China, that benefited from the USA's free-trade ideology and from the "pax americana"). Of course one can always find similarities with previous eras, and predict that the USA will dissolve the way the British Empire did; or not. But Morris thinks that something fundamental has changed in the world with the advent of computers. He doesn't quite explain how, but the fact that everything is being computerized makes him think that the logic of war and peace will never be the same again, and the "pax americana" will be replaced by a "pax tecnologica": technology will make war unwanted. Morris is fascinated by brain-to-brain communications and the technological singularity, but that's all speculation. His point is that war is the rational effect of a situation and that payoff is what matters. In other words, war is more rational than it appears to be. When an empire rules, the payoff to going to war against the empire is very low. He thinks (hopes?) that technology too will reduce the payoff. Peace is not the reason to stop killing each other: the payoff is. It is a cynical, amoral story, but that is also what makes people like Morris believe in the happy ending: self-annihilation is not a rational payoff!
The only (very) annoying feature of this book is that it is all written in miles, feet, pounds and Farenheit degrees, a fact that doesn't increase its scientific credibility.
P.S. At one point Morris writes "The USA has not become a rogue nation". The USA has certainly become one of the most violent societies in the world, with record numbers of murders, and in 2016 a scumbag named Donald Trump became its president.