Why was a treaty between the native Americans and the US needed?

Treaty was needed to serve and satisfy a fundamental need of States to regulate by consent issues of common concern and thus to bring stability into their mutual relations between native Americans and US.

Explanation:

Treaties form the basis of most parts of modern international law. Like International environmental agreements are important because they enable countries to work together to deal with vital environmental issues which are trans boundary or global in nature for e.g. air pollution, climate change, protection of the ozone layer, and ocean pollution.

The federal government in many treaties agreed to guarantee education, health facilities, housing, and other services to native Americans. The United States also agreed to manage and protect inhabitant's resources, such as lands and timber.

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explanation:

under god i have placed my chiest strentgh and the safe guard to my loyal subjects