A gross overstatement. The Creek stayed loyal to the Union. The Cherokee were split between those who supported the Treaty of New Echota and those who did not, with those who opposed the treaty siding with the Union. The Delaware remained loyal. Others did as well.
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.
They fought against the U.S. They had no love for the Confederacy, and the Comanche and the Apache showed.
**The Creek stayed loyal to the Union.**
If they stayed loyal why did they have to give up the western part of their Indian Territory reservation in 1866 just like the other tribes.
https://en.wikipedia.org/wiki/Native_Americans_in_the_American_Civil_War
“At the beginning of the war, Albert Pike was appointed as Confederate envoy to Native Americans. In this capacity he negotiated several treaties, one such treaty was the Treaty with Choctaws and Chickasaws conducted in July 1861. The treaty covered sixty-four terms covering many subjects like Choctaw and Chickasaw nation sovereignty, Confederate States of America citizenship possibilities, and an entitled delegate in the House of Representatives of the Confederate States of America. The Cherokee, Choctaw, Chickasaw, Seminole, Catawba, and Creek tribes were the only tribes to fight on the Confederate side.”
Out west, the Mescalero Apache went on the warpath against the Union when they heard the Confederates were coming. The Confederacy got along well with the Mescalero for a while.
Sons of the Southern Cheyenne, going to Indian schools in St Louis promptly joined the Confederate Army when hostilities broke out. Many of the other tribes, such as the Osages, were split.
It was firmly believed that Confederate agents were slipping into the plains tribes to get them on the warpath against the Union.
Maybe this is why the Indians who were hanged as a warning to others.
Chivigton was a firm believer in confederate agents stirring up the tribes. He referred to the tribes as “Red Rebels” and captured two agents in the camp at Sand Creek.