The Civil War in Indian Territory
From Handy Answer: Native American Almanac
The American Civil War had a dramatic impact on the Five Tribes. A number of tribal members owned slaves and supported the Confederacy. The Choctaw and Chickasaw Nations, whose lands adjoined Confederate Arkansas and Texas, sided with the Confederacy. The three most northerly tribes (Creek, Cherokee, and Seminole) were politically divided but nonetheless made treaties with the South. Loyalist factions continued to favor the North, and many tribal citizens fought on both sides. The Cherokee, Creek, and Seminole each lost as much as twenty to twenty-five percent of their population.