What does Darwin have to do with terracotta warriors, samurai armies, or Napoleon's conquests? Quite a lot.
Or at least this is what I argue in a paper I finished back in April. I anticipated refining it with extra research in the months since then. This hope was not realized. Other projects have consumed my time and I have had to lay aside this paper until I can give it proper attention. But I think the ideas contained in it are interesting, so I will publish it here in working paper form:
For those disinclined to read all 36 pages of it, the paper can be briefly summarized in two statements. The first is an observation, the second an argument.
The observation is this: the military, political, and social histories of Warring States China (471-221 BC), Sengoku Japan (1467-1603), and Early Modern Europe (1453-1816) were all incredibly similar. The origins of all three "warring state" periods are found in the ruins of large empires whose collapse forced hundreds of smaller political units to take control of their own affairs. This was a time of “feudalism” or “fragmented sovereignty,” where politics was personalized, rulers did not exercise a monopoly on violence, and individuals had to navigate conflicting political loyalties. This situation did not last. Governments that once struggled to control the population they ruled and exercised power through the relatives or aristocratic peers of the ruling house soon commanded large and impersonal bureaucracies that directly extracted taxes from and conscripted the service of millions of people. Small aristocratic forces dominated by noble cavalry detachments were eclipsed by gigantic forces of massed infantry. Trade intensified, living standards increased, governments centralized, and most importantly, the number of states dwindled.
I am not the first to note these similarities. The political scientists and historical sociologists Richard Walker, Victoria Tin-bor Hui, Bau Tzong-Ho, Edward Kaiser, and Yong Cai all wrote detailed studies comparing ancient Chinese and Early Modern European politics, while analogies to European social and military history are made often by Hsu Cho-yun and Mark Edward Lewis in their histories of ancient China. Historians Stephen Morillo, George Parker, and Matthew Stavros have described the many parallels between Sengoku and Early Modern European military history in books and journal articles, while John Ferejohn and Philip Striech have detailed the parallels in the institutional development and diplomatic relations of Sengoku domains and Early Modern European states. The comparison between the Senogku era and the Chinese Warring States era is older still; "Sengoku" is the Japanese transliteration of the Chinese characters "zhan guo (战国)" or "warring states." The name was given to the period by Japanese thinkers who wanted to parallel their own country's history with the famous stories of ancient China.
Despite all of this, I haven't found a study that directly compares and contrasts all three eras together.  Doing so myself meant reading dozens of books on each region and synthesizing what data I could gather. I feel most confident in my summary of the Chinese experience. While a few important books are missing from that section of the bibliography, I have read most of the relevant English-language secondary literature and a great deal of the primary sources this literature is based upon (though following standard practice with political scientists, I did not cite this primary material). The weakest section is that on Late Medieval and Early Modern Europe. Much of my work there rests on Thomas Ertman's 1997 study, Birth of the Leviathan: Building States and Regimes in Medieval and Early Modern Europe. This work's conclusions are now disputed, and I ought to revise my argument to reflect this.
What is my argument? To briefly summarize: the parallel changes seen in the Warring States, Sengoku, and Early Modern European state systems (bureaucratic consolidation, territorialization, the rise of mass infantry armies, etc.) should be understood as the results of a Darwinian process. There has been a great deal written over the last three decades on the relationship between warfare and "state formation" or "state building." Most of this literature suggests that it is the pressure of war that creates stronger states and governments--take away the demands of war and we would all be living in feudal baronies today. I believe this general idea is correct, but the causal relationship it suggests is not. If you delve deep into the historical records you often find that the supposed connection between war and government innovation just isn't there. Political consolidation, bureaucratic reforms, and so forth, are often pushed through in times of peace. Many have economic or legal rationales entirely unrelated to external geopolitics. On the flip side, it is not hard to find states in the modern world subject to terrible wars that remain weak and underdeveloped.
My answer to this conundrum is that war does not cause stronger government. It selects for it. The process is not too different from the evolution of a phenotypic trait like camouflage in organisms like the Artic hare. The white snow of the tundra did not cause any individual hare to become white. Rather, hares are far more likely to survive in a snowy environment if they are white. The tundra does not turn the population of hares a different color, but selects for hares whose color better fits the tundra environment until they constitute the entire population. I believe a very similar dynamic was happening over the centuries of warfare in Sengoku Japan, Warring States China, and Early Modern Europe. The parallel institutional developments of these eras look all the world like the process biologists call convergent evolution. If states today (or in other eras of world history) do not converge to the same features that developed in those eras, it is because they face a different set of selection pressures. Instead of searching for a universal causal relationship between warfare and state making, we should be looking to see what constitutes “optimal fitness” in a given state system and whether there are mechanisms which select for these traits.
This is my case in brief. For the full argument, more nuanced and detailed than the gloss above, please see the original paper. All comments will be welcome.
 Though Azar Gat comes close in his discussion of 'feudal war' in his magisterial War in Human Civilization (Oxford: Oxford University Press, 2006), pp. 320-336.