There are many things that Diamond identifies as important in this book. However, the most important is, in my view, geography. Geography was the ultimate factor that allowed Europeans to get the advantages (which Diamond boils down to “guns, germs, and steel”) that made them the dominant power in the world.
To Diamond, the invention of agriculture leads to the development of civilization. Societies that get agriculture earliest become the most powerful societies in the long term. Agriculture developed in Eurasia long before it developed on any other continent.
So why did Eurasia get agriculture first? The main reason is geography. Eurasia had more plant and animal species that were conducive to domestication. Parts of Eurasia had climates that were ideal for agriculture. Eurasia had a long east-west axis that allowed crops to diffuse across the land mass.
In these ways, it was geography that was most important in allowing Europeans to get “guns, germs, and steel.” Geography gave them agriculture, and agriculture gave them the “guns, germs, and steel.”