how to explain the impact of government policies on the great plains
tribes and how the issues of race, culture, economics, and religious
beliefs played a role in the development of those policies?

I think this question might be easier to answer if the scope were
narrowed and more thoroughly defined. What time period are you
concerned with? Which tribes/bands are you including in the Great
Plains tribes? Someone may be able provide you with a summary of
government policies from the Bureau of Indian Affairs or the
contemporary equivalent.