According to Diamond, the fact that Eurasia has an east-west axis is important because it allowed crops and culture to diffuse easily. This meant that Eurasian societies did not have to make everything up for themselves. Instead, they were able to borrow from one another. This allowed them, in a sense, to pool their resources and build societies with more technology.
An east-west axis is important because areas along an east-west line will tend to have similar climates. That means that a plant that is domesticated in one area can be spread to other areas very easily. This is not possible when a continent’s axis is north-south. This meant that any crop domesticated in one area of Eurasia could spread to other areas quite easily. This was not true of places like the Americas.
Crop diffusion is important because it allows societies to get agriculture (and more types of domesticable plants) more easily. Diamond argues that getting agriculture is what allows a society to become strong. Therefore, the east-west axis made it easier for Eurasia to develop agriculture and become strong.