Diamond's thesis is, as he writes in the prologue to Guns, Germs, and Steel:
History followed different courses for different peoples because of differences between peoples' environments, not because of environmental differences between peoples themselves.
The book is intended to get at the ultimate, rather than the proximate, cause of human global inequality. He argues that Europe emerged as a dominant global force because it had a head start in all the things that enable the development of complex, advanced technologies. Put another way, the "guns, germs, and steel" that enabled Europeans to achieve global power were the result of geographic factors. These factors included a number of different things, but they can be boiled down to the rise and spread of agriculture. The development of agriculture enabled Eurasian peoples to settle, creating dense, stratified societies who developed sophisticated technologies.
Crucially, most of these developments occurred by "accident," as it were....
(The entire section contains 3 answers and 869 words.)