This is not Niebuhr's best work. The basic problem is that the work is hardly theological at all. It's an attempt to write the history or historical sociology of empire by someone without any training and based, as far as I could tell, on secondary sources that weren't too fresh even at the time of writing. Given that the book is now more than a half century old, it's amazing anything at all in it holds up. Sadly, not much does.
The basic aim of the book is to place the American and Soviet empires of the Cold War in the historical context of other empires in order to more fully understand the latter and urge America to embrace an imperial role as leader of the free world against it. Quite apart from one's possible political objections to this, almost every stage of the argument is flawed. Although I'm not an expert in this field either, Niebuhr's analysis of ancient empires (essentially that they were all based on a universal religious idea) is misguided. His account of the rise of nations in the west is hillariously teleological: he has "nations" as the dominant form of political organization in Europe by the 16th century, which is at least 300 years too early. His description of early modern and modern European imperialism, especially in Asia, is based on the flawed premise that by the 16th century India and China were technically backward (when it was precisely because they were so far ahead of Europe that Europeans wanted to trade with them). Finally, his argument about Soviet imperialism (that it was based on the universal ideas of Marxism) is, perhaps deliberately, very poor. To give just one example, there's no mention of Western intervention in favor of the Whites in the '20s, which to me seems utterly essential to understand subsequent Soviet foreign policy.
As a source of early American cold warrior thought, I found this book interesting. But it has virtually nothing outside that to offer, except perhaps a cautionary tale about the consequences of not understanding history.