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The five Indian tribes in Oklahoma joined the Confederate cause. Same for most of the Osages.
A few did remain with the Union cause.
The Plains Indians from the Apaches in the South to the Sioux in the north also went on the warpath against the North, except the Pawnee and one small weak band of Sioux.

When Chivington hit the Southern Cheyenne camp at Sand Creek he captured two Confederate agents who were there to make sure the tribe again went on the warpath after the winter was over.
After the war was over, the Five Civilized Tribes in Oklahoma were forced to cede to the US the western half of their lands.

Actually they did. Most of the troops came from southern state militias.

Excuse me, but back then all troops were mustered state by state.

You may remember all the battles listing military units as 29th Alabama, 22nd Massachusetts, 7th New York Regiment, 8th Texas Cavalry, etc.

On May 26, 1830, Congress passed the Indian Removal Act, which Jackson signed into law. The Act authorized the President to negotiate treaties to buy tribal lands in the east in exchange for lands farther west, outside of existing U.S. state borders.[85] The passage of the bill was Jackson's first legislative triumph and marked the Democratic party's emergence into American political society.[85] The passage of the act was especially popular in the South where population growth and the discovery of gold on Cherokee land had increased pressure on tribal lands.
The state of Georgia became involved in a contentious jurisdictional dispute with the Cherokees, culminating in the 1832 U.S. Supreme Court decision Worcester v. Georgia. In that decision, U.S. Supreme Court Chief Justice John Marshall, writing for the court, ruled that Georgia could not impose its laws upon Cherokee tribal lands.[87][88] [e]
Jackson used the Georgia crisis to broker an agreement whereby the Cherokee leaders agreed to a removal treaty. A group of Cherokees led by John Ridge negotiated the Treaty of New Echota with Jackson's representatives. Ridge was not a widely recognized leader of the Cherokee Nation, and this document was rejected by some as illegitimate.[89] A group of Cherokees petitioned to protest the proposed removal, though this wasn't taken up by the Supreme Court or the U.S. Congress, due to delays and timing.[90]
The treaty was enforced by Jackson's successor, President Martin Van Buren, who sent 7,000 troops to carry out the relocation policy. Due to the infighting between political factions, many Cherokees thought their appeals were still being considered when the relocation began.;[91] subsequently, as many as 4,000 Cherokees died on the "Trail of Tears" in 1838.[92]

https://en.wikipedia.org/wiki/Andrew_Jackson

None of it would have happened without Jackson. The very stately home of Major Ridge, the father of John Ridge, still exists in Rome, GA where I grew up.

Yes, a Cherokee Chief built and owned this house.

In 1839, after removal to Indian Territory, opponents assassinated the Ridges, Boudinot, and other Treaty Party members for their roles in the land cession and to eliminate them as political rivals. Stand Watie survived an attack.

Stand Watie later became a Confederate States Army General of the 1st Cherokee Mounted Rifle brigade and was the last Confederate General to surrender to Union troops on June 23, 1865, two and a half months after General Lee surrendered on April 9, 1865.