As Pagden writes, Peoples and Empires is fundamentally about "what drives people into contact—and conflict—with each other." It is also fundamentally about power. These three themes, contact, conflict, and power, inform the entire book.
European empires had a number of effects on the peoples that lived within them. When Pagden writes about "contact," he does not simply refer to contact between the colonizers and the colonized, though this is of course important. He also means that empires as units of administration had the effect of bringing diverse cultures and peoples into contact with each other. European empires were, Pagden writes, "universal, cosmopolitan societies." Alexander the Great, for example, aimed to bring together East and West, assimilating the many cultures within the old Persian Empire with Greek civilization, which had previously dismissed all non-Greeks as barbarians.
At the same time, the very act of cultural contact led to conflict. Pagden writes that nationalism "swept the once massive imperial edifices away" in the twentieth century. One of the reasons they did so was that people began to turn to a specific cultural identity that was seen to be at odds with that of the polyglot empire. Empires represented an affront to this identity, which, it must be added, was as constructive and fictive as the ideology of empire itself. Pagden writes that this led to further conflict, as it was much easier for various peoples in the British Empire to recognize that they were "not English" than it was for them to assimilate to each other.
As for power, Pagden's narrative makes it clear that contact always occurred in ways that were imbalanced and exploitative. African slaves, for example, became part of an American creole culture that they deeply influenced, but they did so against their will. European power battered its way into Chinese markets, and continues to influence governments of former colonies around the world. Whether empires were based on cults of personality, like that of Alexander, or on vast administrative bureaucracies, they were fundamentally about power, however cosmopolitan and diverse they were.