When Europeans reached America, they dominated the battles eg. when Pizarro faced the Aztecs in South America - 168 Spanish soldiers defeated thousands of Aztecs in a quick and dirty battle. They had horses guns and cannons, technology that was far superior to anything natives had. Generally more than 90% of native population was wiped out even before the battles took place by coming in contact with the germs that Europeans were resistant to while natives were not. Why was it that Europeans had better technology and germs? Why not the other way round?
The key event in human history, from which all the development followed - was start of food production. This started in fertile crescent area around 10000 years ago. This area was particularly suitable because it had the temperate climate, wild forms of the crops which are were developed for farming, wild ancestors of animals suitable for domestication. Nearly everywhere else in the world, this technology was imported along with the seeds, animals etc. Food production enabled mankind to support higher population density and emergence of organized society.This led also to invention of writing in the fertile crescent , which was also adopted by all other civilizations over the world. Germs and germ resistance developed as a result of humans living in closed societies and close to domesticated animals (like Influenza virus came from the domesticated pig)
Everywhere else in the world, no suitable crops were available. For example in America, only suitable crop was corn , there was no wheat, rice, millet, barley etc. and even the wild ancestor of corn was 6 cm so it took thousand of years of selection to develop corn to its modern form. Also there were no animals worth domesticating. Only Llamas, which could not be used over all of america but only some specific parts.
Technology traveled faster along the east west axis in Asia. Because same crops could be used in similar climate. While on the north south axis the propagation of technology was very slow. America and Africa being aligned North-south was also a big disadvantage.
Human brain is similarly developed in all major races. Brain evolution stopped around 40000 years ago.It was only a matter of chance and circumstances why these technologies were developed in Asia and traveled to Europe through land. Everywhere else humans showed same kind of ingenuity in understanding their environment - crops and animals. But it was not possible to achieve a lot with what was available, so a lot of races did not develop or even went back to hunting gathering lifestyle.
Technologically, most of Asia and Europe was at a similar level at around 1500 AD. For the rest of the world, differences in technological development can be explained purely from the variables outside human control. Since then Europeans made amazing progress reasons for which have not been explained as convincingly as for period before that. Reasons presented are mostly idiosyncratic - as in Europe benefited from being fragmented and plural while China was unified and not plural. Diamond proposes that there is an optimum fragmentation/unification level. In any case, it is not the main purpose of the book to explain the differences created in the last 500 years, which have already started to be normalized and probably will disappear in next 500 years. The book presents a very long term view of human history, one which has presented insightful arguments and enriched me immensely.