In "The Unbreakable Code" why did John's grandfather speak sternly when talking about the Navajo language

He was the grandson of a Navajo code talker and he had a language that once helped save his country. Grandfather wanted to join the marines for several reasons: He did not like boarding school because they cut his hair and didn't allow him to speak Navajo. In the marines, they wanted him to speak Navajo.

Explanation:

Hope this helps:)

these civilizations were located around fertile rivers, where they could produce an abundance of crops