A compelling argument for realism. Thayer first demonstrates great command of the literature in narrating the shift from classical to structural realism. His summary of realism could be a great introduction to the subject outside of its purpose in his argument. The essence of his argument for the evolutionary origins of war and ethnic conflict comes by way of evolutionary sociobiology.
For me this book sits at a crossroads of interests in political philosophy and the implications of evolutionary theory for human social behavior.
I did not agree with nor totally understand Thayer's treatment of Clausewitz. After spending a great deal of time demonstrating how evolutionary theory explains politics (and hence war), Thayer concludes war cannot be the mere continuation of politics. We must instead, he insists, come to a broader definition (one that fits all forms of "warfare" in the animal kingdom). I, however, see Clausewitz's definition of war as a continuation of politics to fit Thayer's realism, insofar as he is merely explaining the biological determinants of politics. There is nothing in Clausewitz to deny such an explanation of politics (in fact he is notably absent a definition of politics), and furthermore Clausewitz operated in the realist frame.