Diamond argues that Europeans were able to conquer the natives of the Americas because of geographical luck. The Europeans had had better geographical luck and, therefore, they had the “guns, germs, and steel” that allowed them to conquer.
The Europeans had been born in a continent that had had agriculture for much longer than the Americas had. Their agriculture had been more effective as well. This is partly because there had been more species of large, domesticable animals in Europe than in the Americas. It is also because the types of grain available in Europe were more nutritious than those in the Americas.
What this meant is that Europe was able to have larger, denser populations from much earlier than was the case in the Americas. This gave them the ability to create more kinds of technology. It also gave them the massive populations and the proximity to animals that led to the existence of epidemic diseases. It was the technology and the diseases that were mainly responsible for the conquest.
Thus, the Europeans had technological advantages and the advantages conferred by their diseases. These factors, which were caused by geographic luck, allowed them to conquer the Americas.