Gat denies that the "immanent logic" of evolutionary reasoning has any "transcendent measurement." Consequently, "the evolutionary logic in itself has no normative implications." Evolutionary science can tell us about our evolved natural desires, which we should take into account in organizing our individual and social lives. But, then, he says: "We may choose to follow such predispositions or rebel against them. . . . The human brain . . . may come up with more satisfactory arrangements."
This suggests that Gat is adopting a Kantian dualism of the sort that was defended by Thomas Huxley and later adopted by many evolutionary psychologists. If so, then evolutionary science can explain the "natural facts" of human desires but not the "moral values" of human culture and human reason. Human morality requires a "transcendent measurement" that is beyond natural experience. Moral normativity transcends scientific empiricism.
But how exactly does this dualism work in explaining war? In explaining the importance of war in the earliest agricultural societies and in the emergence of the state, Gat indicates that war in these societies was motivated not only by material interests but also by abstract ideas, including transcendent moral and religious views of the cosmos. The invention of writing and the development of literate culture allowed the emergence of religious and philosophic texts that supported grand worldviews and transcendent conceptions of cosmic order. These cosmic belief systems became important for legitimizing war and state power.
Here the moral, religious, and philosophic advances made possible by a literate culture don't deny but rather reinforce the natural desires that lead to war. And yet, Gat also sees a more radical attitude emerging--particularly, in the "Axial Age"--that supports normative ideals that rebel against natural desires. Greek philosophers began to argue that the fullest human happiness was found not in the restless pursuits of war but in the leisured life of pleasure and intellectual activity (438-40). It was not clear, however, that such a life of purely peaceful leisure was possible for all human beings. (Gat might have noted that even in Plato's "city in speech," in the Republic, there had to be warriors.)
Gat points to the asceticism and pacifism of Buddhism as the most radical attempt to rebel against the natural desires of evolved human nature. But he admits that the ascetic ideal was not very successful in stopping war.
Rather than the normal balancing of desires against each other and against possible costs, asceticism involves an attempt at a wholesale suppression of desire and constitutes a rebellion against the evolution-shaped human motivational system. Indeed, it is for this very reason that asceticism has remained marginal in human society, because it has gone against our most deeply rooted, innate predispositions. Seriously attempted only by a very small minority, it has mostly served as an unfulfilled option, a spiritual yearning, a creed, or, at most, a disciplinary constraining factor, for those among the vast majority of people who have felt tortured by the frustrations of desire. (440-41)
Gat goes on to note that ascetic religious believers--in Christianity, Islam, and Hinduism--have generally been the best warriors. And even Buddhism has had a tradition of warrior-monks. The history of how the martial arts were cultivated in Buddhist monasteries, and fully integrated into Zen Buddhism, is a remarkable indication of how even the most radical asceticism fails to conquer the natural desires that lead to war.
To me, this casts doubt on Gat's dualistic claim that a normative morality of transcendent ideals is beyond the empirical claims of evolutionary science. The failure of philosophic rationalism and religious asceticism to fully suppress the natural desires of our evolved human nature suggests that human flourishing must conform to those desires. And, if so, we cannot escape the natural desire for courage in war, and thus the yearning for a normative morality of universal love and perpetual peace must always fail.
Some posts on related points can be found here, here, here, and here.