What was the source of an increase in Native American tribal unity in
the late 1700s?

The source of an increase in Native American tribal unity in the late 1700s is "common enemy"

Hence, in this case, the correct answer is "Common Enemy which is the American colonists."

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The resistance towards the European in America.

Explanation:

Native Americans resisted the efforts of the Europeans to gain more of their land and control through both warfare and diplomacy. Many of the Native Americans came together in the late 1700s as colonists began to expand their boundaries into Indians. For European settlers, owning land was extremely valuable because it meant wealth, power and position in society. The hostile between the two groups began when settles also ignored the treaties signed between Americans and Natives Americans. In seems, Native Indians suppressed and thrown out of their land, which they considered being holy where they have lived for years as have their ancestors.

fourteenth amendment