When Minnesota became a state in 1858, the leftover area between the Missouri River and Minnesota's western boundary fell unorganized. When the Yankton Treaty was signed later that year, ceding much of what had been Dakota land to the U.S. Government, early settlers formed an unofficial provisional government and unsuccessfully lobbied for United States territory status. However, it wasn't until three years later when soon-to-be-President Abraham Lincoln's cousin-in-law, J.B.S. Todd, personally lobbied for territory status that Washington formally created Dakota Territory. The name refers to the Dakota branch of the Sioux tribes which occupied the area at the time. It became an organized territory on March 2, 1861 including much of present-day Montana and Wyoming. By 1868, creation of new territories reduced Dakota Territory to the present boundaries of the Dakotas.
After becoming an organized territory, the population increased very slowly during the early years and then very rapidly with the "Dakota Boom" from 1870 to 1880. Veterans of both the Union and Confederate armies moved to Dakota Territory. The 1893 Veterans Census shows 5875 Union Soldiers and 69 Confederate Soldiers.