Kennedy argues that the strength of a Great Power can be properly measured only relative to other powers, and he provides a straightforward and persuasively argued thesis: Great Power ascendancy (over the long term or in specific conflicts) correlates strongly to available resources and economic durability; military overstretch and a concomitant relative decline are the consistent threat facing powers whose ambitions and security requirements are greater than their resource base can provide for (summarized on pages 438–9).
Throughout the book he reiterates his early statement (page 71): "Military and naval endeavors may not always have been the raison d'être of the new nations-states, but it certainly was their most expensive and pressing activity", and it remains such until the power's decline. He concludes that declining countries can experience greater difficulties in balancing their preferences for guns, butter and investments.[2]
Kennedy states his theory in the second paragraph of the introduction as follows.
The "military conflict" referred to in the book's subtitle is therefore always examined in the context of "economic change." The triumph of any one Great Power in this period, or the collapse of another, has usually been the consequence of lengthy fighting by its armed forces; but it has also been the consequences of the more or less efficient utilization of the state's productive economic resources in wartime, and, further in the background, of the way in which that state's economy had been rising or falling, relative to the other leading nations, in the decades preceding the actual conflict. For that reason, how a Great Power's position steadily alters in peacetime is as important to this study as how it fights in wartime.[3]
Kennedy adds on the same page.