Final answer:
The conflicts between the United States Army and Native American tribes following the Civil War are known as the Indian Wars, characterized by significant opposition and cultural transformation of Native American life. Hence, option (a) is correct.
Step-by-step explanation:
Following the Civil War, the conflicts between the United States Army and Native American tribes are known as the Indian Wars. This period was marked by violence and upheaval as the US pursued westward expansion, leading to many clashes with various Native American tribes.
Key figures and tribes such as the Lakota Sioux, the Apache, and the Navajo, led by leaders like Crazy Horse and Red Cloud, fought against the infringement of their territories and lifestyles. However, despite some victories, these wars often resulted in the displacement of Native American tribes and their confinement to reservations. The transformation of their cultures and suppression of traditional ways of life were lasting consequences of these conflicts.
The federal government's policies were often justified by citing the military alliance of some tribes with the Confederacy during the Civil War or the skirmishes with US troops as evidence of Native American hostility. Reforms and changes towards American-indigenous relations were influenced by these events and led to shifts in how the US interacted with and controlled Native American populations, especially through the imposition of civilization policies aimed at assimilation.