The thesis that Diamond is trying to prove in this book is that geography determined the major trends in human history. He is trying to show that it is geographical luck, and not racial or cultural superiority, that led to the dominance of Europeans over other societies of the world. Diamond starts with "Yali's question," in which he is asked why white people are so much richer and more powerful than New Guineans. In this book, he attempts to show that geography causes these differences. On p. 25 in the paperback edition of the book, Diamond provides a succinct statement of this thesis when he says that some people dominate and others are dominated
...because of differences among peoples' environments, not because of biological differences among peoples themselves.