During Sam Houston's First Administration the duties of the Texas Rangers included —

Repelling the Native Indians

Explanation:

During the period of settlement in Texas, many native Americans carried out series of attacks toward the settlers since many of them occupied the ancestral land that previously belonged to the Natives.

To deal with this, Sam Houston utilize Texas Rangers to repel the attack from the Native Americans and protected the white Settlers' interest. This Rangers' duties was carried out until the year of  1838.

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