How did the Civil War affect the Native Americans?
The main impact that I can think of with regard to this question is that the Civil War helped to cause conflicts between Indians and the United States.
The Civil War, of course, took up a lot of government resources. Because of this, Native Americans in the Midwest weren't paid the way the treaties said they had to be. This led to conflicts between the US and the Indians, including a famous incident in Minnesota that ended with about 40 Sioux being hanged. It was also during the Civil War that the infamous Sand Creek Massacre happened.
There were also Indians who fought in the Civil War, mainly on the side of the South. But this had little effect other than on the individuals who fought.
Mainly, then, the Civil War helped cause conflicts to erupt because the US Government didn't have the resources to either keep the Indians happy or keep them suppressed.
The Civil War brought forth the opportunity of hope for many Native Americans. The soldiers that were formerly assigned to the forts, that practiced a strong hold on the Indian lands, were diverted to fight in the Civil War. The Indians in some areas hoped this would provide them with the chance to retake some of their lands. Violence broke out among the Native Americans and the settlers and soldiers as they tried to regain the land. Around 3, 500 Native Americans served in the war, most being from the Cherokee nations. Indians living in the North and East, particularly those living in the path of the war, had to make their own decision whether to participate in the war or leave the area. Indians that were living on reservations and other areas were subjected to fewer supplies as the funds formerly used to provide for their support were being allocated to the war effort.
The Civil War actually delayed the fate of Native Americans, for it occupied the US military for over four years, years which would have been spent trying to control and colonize the Indians. Once the war broke out, many experienced military leaders were pulled from the West, and most were Southerners.
Additionally, the Civil War and its final outcome renewed the sense of entitlement and power that many Americans felt following the Revolution and the War of 1812. After fighting such a horrific, bloody war, Americans certainly felt that the conquest of the entire northern continent was their right. Certainly the Indians would not be permitted to threaten the expansive lifestyle that over 600,000 Americans had died to protect.
Another important aspect of the Civil War was the military experience it yielded. Many men were novices who eventually mastered their trade by the end of the war. These hardened men were sent West after the war, and their mentality was completely changed by the horrors they had seen in the war. Additionally, after previously facing fellow countrymen in battle, these soldiers were now facing an enemy of a different color, an aspect that allowed them to justify the horrfic actions that took place during the Indian struggles that followed the Civil War.