Buck Creek Camp was southwest of Ft Smith in the Choctaw Nation on, of course, Buck Creek. At the time of this letter, Pike had resigned and Cooper was in command of the Indian Troops in the Indian Territory. When the Wichita Agency was attacked by "Kansas Indians", Pike gathered a few companies of Texans to investigate and was accused of claiming to be in command of the Indian Territory. The defeat Cooper refers to is probably Maysville (on the Arkansas-Cherokee line) meaning the defeat at Ft Wayne -- Maysville was about 4 miles northeast of Ft Wayne and Hindman refered to the battle as Maysville.
See OR, Series 1, Vol. 13, Page 918 for Holmes description of the events leading to Pike's arrest.