The basic question that Diamond is trying to answer in this book is that of why some cultures or countries became strong and rich while others did not. Specifically, he is asking why the Europeans were able to go out and conquer other people back in the age of exploration rather than having those people (like the Incas) go and conquer them.
Diamond's basic thesis is that it is all down to luck. He says that some areas were luckier, geographically speaking. They had geographical advantages that allowed them to develop agriculture first. This allowed them to develop the "guns, germs, and steel" that allowed them to go and conquer the world.
The thesis of Diamond's book is that, in the words of the author:
History followed different courses for different peoples because of differences among people's environments, not because of biological differences between peoples themselves (25).
He claims that this view is largely eschewed by most academics. Historians in particular think it diminishes the importance of human action, and argue, according to Diamond, that it is deterministic. In other words, some say that Diamond's view seems to suggest that a people's environment determines everything else about their development, from their culture to the technology they develop. Diamond aspires to write what he calls a "unified synthesis" of a range of disciplines, including human genetics, history, archaeology, evolutionary biology, and epidemiology (26). To put it another way, he says that human history must be established as a "historical science, on a par with recognized historical sciences such as evolutionary biology, geology, and climatology" (32).
For the lay reader, one who is not familiar with modern scholarship on the development of human societies, one facet of Diamond's argument is especially important. He wants to underscore the idea that it was not the cultural, moral, or racial superiority of Europeans that enabled them to develop the technology that enabled them to conquer and influence peoples around the world. Rather, it was accidents of geography that facilitated the development of crops, and even pathogens that made it possible for them to do so. Diamond believes that historians have over-emphasized the importance of culture without considering the scientific foundations of its origins among different peoples.
Of course, Diamond's thesis is an answer to what he calls "Yali's question," which a New Guinean man by that name posed to him while he was doing fieldwork there:
Why is it that you white people developed so much cargo [i.e., material goods] and brought it to New Guinea, but we black people had so little cargo of our own (14)?
It was not because of any relative shortcomings, biological, cultural, or otherwise, that Europeans were able to develop more "cargo."
Jared Diamond introduces Guns, Germs, and Steel with a question asked him by Yali, an aboriginal New Guinean leader:
"Why is it that you white people developed so much cargo and brought it to New Guinea, but we black people had little cargo of our own?"
The term "cargo" means manufactured goods and other rare and expensive objects. As Diamond reflects on this question, he also becomes curious about the reasons for unequal development in various areas of the world. He mentions that it obviously has nothing to do with hard work or natural ability, as there are both smart and less intelligent and hard-working and lazy people in all cultures and regions.
Diamond's main thesis is that geography and physical environment have, to a large degree, placed constraints upon how societies can develop. He investigates various topics such as the availability of domesticable plants and animals, the barriers to travel to encounter and trade with cultures at the same latitudes, and other characteristics of natural resources and environment to argue his main thesis, that it was differences in external environmental factors, rather than intrinsic superiority, that led certain civilizations to become wealthy and powerful.