Around twenty thousand Native Americans fought in the Civil War. But on which side? Did they fight for the Union in solidarity with the enslaved people of the South? Or did they fight for the Confederacy to get back at the United States for their own mistreatment? The truth is, they fought for both sides, with way more volunteering for the Confederacy, but for neither of the reasons above. There were many reasons a Native American joined one army or the other, which just shows us the danger of generalizing a large group of people, and the danger of assigning modern motives to people in previous centuries.
After the Civil War, Native Americans fought in all US wars, and continue to this day to sign up in higher percentages than other ethnic groups. Read an overview of Native Americans in military uniforms, with a good look at the Navajo Code Talkers, at Messy Nessy Chic.