On March 2, 1861, the Congress of the United States created the Dakota Territory, which consisted of the present-day states of North Dakota and South Dakota, and most of Montana and Wyoming. The name was taken from that of the Dakota. In their language, Dakota means "allies." In 1863 the size of the territory was reduced to the area of North and South Dakota. With increasing immigration and settlement, by the late 1870s Dakotans felt inadequately represented by territorial status and began pushing for statehood, either as one state or two. By the late 1880s, northern Dakota had 19 0,000 residents and southern Dakota had over 340,000, population sizes that justified statehood.

The Omnibus Bill of February 22, 1889, passed by Congress, authorized framing of constitutions in Montana, North Dakota, South Dakota, and Washington. On November 2, 1889, both North and South Dakota were admitted to the United States. Since President Benjamin Harrison did not want to show favoritism, after he signed the Act of Admissions papers for North Dakota and South Dakota, he mixed them up. Therefore, their order of admissions is listed alphabetically, with North Dakota the 39th state and South Dakota the 40th state.