The issue was contentious for the legislature between the creation of the Nebraska Territory (which Montana was a part of) in 1854 and the outbreak of the American Civil War in 1861.
However, there was apparently a particular acceptance of African Americans in the Nebraska Territory when they first arrived en masse.
According to a publication by the Federal Writers Project,[1] In the Territory of Nebraska the fight to exclude slavery from within the territorial boundaries spread from the Senate to the press and to the pulpit.
Even among the slaves in the South the word spread that here was a place where the attitude toward Negroes was tempered with tolerance.
York, an enslaved African American held by William Clark, traveled and worked with him in 1804 and 1806 as part of the famous Lewis and Clark Expedition's exploration of the Missouri River lands.