How are Afghanistan's race, ethnicity, and tribes important to the story "The Kite Runner"?

The different tribes of Afghanistan set up one of the major conflicts in the book. Amir is a pashtun, a member of the privileged ruling class of the country. Hassan is a hazzara, a class of servants who are not only socially different but also look different form their pashtun counterparts. This causes conflict between the two, especially in Amir’s eyes, as he is clearly friends with Hassan, but at the same time Hassan is a servant to Amir. Amir is reluctant to treat Hassan like a friend when others are around, or to admit that Hassan is in fact his friend. Much later in the book, when Amir returns to Afghanistan, the country has been taken over by the Taliban, who are mostly Pashtun, and Assef, who as a child said that Afghanistan was a country for the Pashtuns and will one day be for the Pashtuns again, is a high ranking official.