Fort Abercrombie (ăbˈərkrŏmˌbē, –krŭmˌbē) [key], U.S. army post on the west bank of the Red River, at Abercrombie, N.Dak.; est. 1858. Built to protect settlers in the Red River valley from attacks by the Dakotas, the fort played an important role in opening the Dakota Territory to settlement. It was twice attacked unsuccessfully by the Sioux in 1862. With the signing of a treaty with the Ojibwa and the Sioux there in 1870, the fear of Native Americans declined and the fort was abandoned in 1877. Part of the former installation is now a state park.