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:

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native americans suffered high rates of poverty and unemployment.

during the late 1960s and early 1970sthe need for political militancy grew among the native americans. throughout the 1960s, they had been the nation's poorest minority group, the most deprived group, according to practically every socioeconomic measure. in 1970, the indian unemployment rate was 10 times the national average, and 40 percent of the native american people were below the poverty line.