Is it best to describe Diamond as a cultural or an enviromental historian?
Diamond is emphatically an environmental historian. The main thrust of his book is aimed at dispelling the idea that societies developed differently because of cultural or intellectual deficiencies of allegedly "primitive" cultures. To emphasize the point, Diamond argues that Eurasian cultures developed technologies first because of geographic advantages. These advantages helped them to become agriculturalists long before other peoples, and as Diamond reasserts throughout the book, agriculture is the most important factor in the development of the factors that have enabled some societies to rise to power over others. In his search for what he calls "ultimate factors" in human development, he concludes that these factors are overwhelmingly related to environment, not culture. As he says in his prologue:
The whole modern world has been shaped by lopsided outcomes. Hence they must have inexorable explanations, ones more basic than mere details concerning who happened to win some battle or develop some invention on one occasion a few thousand years ago.
Battles and even inventions are proximate causes, and one must look past them to attempt an answer to questions as fundamental as those posed by Diamond in Guns, Germs, and Steel.
Source: Diamond, Guns, Germs, and Steel, 25.