Why were the Five Tribes grouped together?

During the American Civil War most tribes were divided between supporters of the Union and the Confederacy, providing soldiers for each army. ... When transcontinental railroads were built through Indian Territory and the settlement of adjoining states increased, the Five Civilized Tribes lost their independence.

Explanation:

in 1897, pershing became a tactical officer at west point, and that's where he was given the nickname "black jack." pershing's first name is "john," for which "jack" is a nickname, and he was a white man who commanded black troops. hence "black jack".