The Civil War in Indian Territory
At the beginning of the Civil War, Indian Territory had been essentially reduced to the boundaries of the future State of Oklahoma, and the primary residents of the Territory were members of the Five Civilized Tribes plus a number of Plains tribes that had been relocated to the western part of Indian Territory on land leased from the Five Civilized Tribes. In 1861 the US abandoned Fort Washita, leaving the Chickasaw and Choctaw Nations defenseless against the Plains tribes. Later the same year the Confederate States of America signed a Treaty with Choctaws and Chickasaws. Ultimately, all of the Five Civilized Tribes, as well as several other tribes that had been relocated to the area, signed treaties of friendship with the Confederacy.
During the Civil War, Congress gave the President the authority to, if a tribe was "in a state of actual hostility to the government of the United States... and, by proclamation, to declare all treaties with such tribe to be abrogated by such tribe"(25 USC Sec. 72).
Prior to the Civil War, the Pottawatomie Massacre (May 24-25, 1856) was one of the many bloody episodes in Kansas preceding what came to be known collectively as Bleeding Kansas.