As part of the 7th Infantry, Woodruff took part in the Great Sioux War of 1876, assigned as regimental adjutant and commander of a battery that consisted of one Napoleon gun and two Gatling guns. After the Battle of the Little Bighorn, Woodruff continued to be interested in the events surrounding the fight and the personalities involved. In addition to maintaining a correspondence with other participants in the campaign with whom he discussed his experiences, he also communicated with several writers and historians who researched the subject.