mccook resigned from the volunteer service in october 1865 and was commissioned lieutenant colonel of the 26th infantry in march 1867. he served in texas, mostly in garrison duty, until 1874. from 1875 to 1880, he served as the aide-de-camp to the general-in-chief of the u.s. army, gen. william t. sherman. from 1886 to 1890 (except for brief terms of absence), he commanded fort leavenworth, kansas, and the infantry and cavalry school there. he then commanded the department of arizona from 1890 to 1893 and the department of colorado from 1893 to 1895.