Kennedy offers a history of major world powers between 1500 and 2000, ending with some speculation on how the world stage will shift based off his historical conclusions. A key point Kennedy makes is that the decline of great powers is marked by military spending over-extending its economic base, setting the power on an unsustainable path. Kennedy notes that in extended conflicts, victory has consistently gone to the nation(s) with greater economic power and that a heavy focus on military spending necessarily takes away from economic investment, especially when a nation is already strained. This was clearly a suggestion that the US was (and is) overextended and that attempts to police world events speed its decline.

Kennedy discusses the rise and fall of great powers as something of a natural cycle. He notes that the ability to exert massive military and economic influence on the world sets powers up to over-extend themselves and then enter periods of decline. He treats the decline of the United States as a fact, simply arguing that the decline can be managed in ways that make it a relative decline from a dominant power to a major player; conversely, the United States' policies could remain unsustainable, and the nation could lose the ability to exert such significant influence on world events. Kennedy shows that maintaining a sprawling empire across the globe is fundamentally unsustainable and that the sprawl of US military bases and operations clearly represents such an empire.

Kennedy pays little mind to human agency but does not completely discount it. He notes that military strategy can be decisive in shorter conflicts where opponents are somewhat equally matched or where economically unsustainable practices don't have time to catch up to militarily-focused powers. He also notes that the economic state of a power will necessarily affect the day-to-day life of its subjects and that social upheaval goes hand in hand with imperialism.