North Dakota covers 70,704 sq mi; its capital is Bismarck. The Missouri River crosses it; the Red River forms its eastern boundary. There is evidence of prehistoric inhabitation throughout the state. At the time of European contact, it was inhabited by various tribes of Plains Indians. It became part of the U.S. with the Louisiana Purchase of 1803. The northeastern corner was added by a treaty with Great Britain in 1818. In 1804–05 the Lewis and Clark Expedition wintered there among the Indians. In 1861 it became part of the Dakota Territory. Separated from South Dakota, it was admitted to the Union in 1889 as the 39th state. In the 20th century North Dakota's history was marked by the increasing mechanization of agriculture, the enlargement of farms, and the loss of a rural population. In the 1950s it became an oil-producing state, and in the 1960s air bases and missile sites were built there. Its larger cities include Fargo, Grand Forks, and Minot.
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