The battalion skirmished at the Battle of Kennesaw Mountain and took part in an attack that overran a line of Confederate rifle pits on July 4.

Fetterman subsequently was assigned as Assistant Adjutant General of the 2nd Brigade and reported on a successful attack at the Battle of Jonesborough on September 1.

In November 1866, the regiment was stationed at Fort Phil Kearny, tasked with protecting immigrants traveling to the gold fields of Montana Territory along the Bozeman Trail.

"[5] On December 21, 1866, a large band of Cheyenne and Sioux – which included Crazy Horse – under the leadership of Red Cloud attacked a wood train near the fort.

Ignoring his orders not to venture beyond Lodge Trail Ridge (out of sight and support distance from the fort), Fetterman pursued a small band of Sioux and was lured into an ambush.

It led to the dismissal of Fetterman's commanding officer, Henry B. Carrington, who was initially blamed for the disaster, but was eventually exonerated.