Terry,
Would you take a look at his letter with the heading:

Camp in the field, Cherokee Nation
August 13th, 1863

In this letter he states, "We have been on the march most of the time since the 22nd Aug. chasing the rebels we could only get near enough to skirmish with their rear guard a couple of days. Our camp is about half way between Ft. Blunt & Ft. smith on the Arkansas river. Some of our troops are at Ft. smith. Which place is in our possession."

There is an obvious problem with the dates -- he mentions being on the march since Aug. 22nd but the letter is dated Aug. 13th. Also, Ft Smith was not occupied by Union forces until Sept 1st.

Could he have been writing on Sept. 13th instead of August?