Named for a popular Union general killed in the Civil War, Fort Phil Kearny was established at the forks of the Big and Little Piney Creeks by Col. Henry B. Carrington of the 18th U.S. Infantry in July, 1866. The Mission of this fort and two other posts along the Bozeman Trail--Forts Reno and C.F. Smith--was three- fold: to protect travelers on the Trail; to prevent intertribal warfare between Native Americans in the area; and to draw attention of Indian forces opposed to Euro- American westward expansion away from the trans-continental railroad construction corridor to the south.
All three Bozeman Trail forts were stockade fortifications, with Fort Phil Kearny being the largest. Enclosing seventeen acres, the fort wall was eight feet high, 1,496 feet in length, and tapered in width from 600 feet on the north to 240 feet on the south. More than four thousand logs were used to erect the stockade, while over 606,000 feet of lumber and 130,000 bricks were produced in 1867 alone for the extension building construction.
During its two year existence, Fort Phil Kearny was the focal point of a violent war between the U.S. Army and the Sioux, Cheyenne, and Arapaho Indians opposed to intrusions into the last great hunting grounds on the Northern Plains. Besides the Fetterman and Wagon Box battles, many smaller fights took place in the area.
By 1868, the Union Pacific Railroad had reached a point to the west where travelers could bypass the Bozeman Trail route by going to Montana through Idaho, thus making the Bozeman Trail forts expensive liabilities. In the Treaty of 1868, the United States agreed to close the forts and the trail. Fort Phil Kearny was abandoned by the Army in early August, 1868, and burned soon afterwards by the Cheyenne. In 1963, Fort Phil Kearny was designated a National Historic Landmark. Today, portions of the fort site and the Fetterman and Wagon Box battlefields are included within the Fort Phil Kearny State Historic Site boundaries.