In Guns, Germs and Steel, what chapter in Part 4 was Diamond's weakest argument?

One of the questions that I still have about the brilliant
thesis that Diamond presents in this text concerns the dominance of the Chinese nation
that he explores in depth in Chapter 16. In this chapter, Diamond goes into depth to
explain why the Chinese culture was so strong and powerful, and how it, in his words,
"steamrollered" other cultures around it. Throughout this chapter, the importance of
China and how it shaped its immediate geographical area, especially towards the South
West, is made amply clear, and Diamond himself says that the role of China in history
was "disproportianate." I would have valued more information as to why it was that
China's influence was only directed towards the South-West and why China itself did not
come to be the leading power block of the world in the way that Europe did. Whilst
Diamond does attempt to address such issues in his Afterward, this was a neglected area
that could have been delved into further, in my opinion.