You can find the answer to this question in the Prologue. There, we can see that Diamond is trying to prove that some human societies have come to dominate others simply because of geographical luck.
The book is based on "Yali's question," which is, essentially, a question about why Europeans have come to dominate the world. Diamond dislikes the answers that are typically given to explain this phenomenon. These answers tend to focus either on racial characteristics or on culture. Diamond feels that these explanations are wrong and that they unfairly denigrate "primitive" cultures. In the book, Diamond tries to prove that geography is the answer to Yali's question. He is trying to prove that Europeans came to dominate the world because they were lucky, not because their race or their culture was superior.