Congress created the Arkansas Territory from the larger Missouri Territory in March 1819. Originally comprising all of the current state of Arkansas and most of the current state of Oklahoma, it was reduced in 1824, when the western half was separated, and again in 1828, when a smaller western portion was removed, thereby establishing the current state borders. Arkansas retained its territorial status until it achieved statehood on June 15, 1836. Arkansas Post was the territorial capital until 1821, when it moved to Little Rock.
“An Act Establishing a Separate Territorial Government in the Southern Part of the Territory of Missouri,” 2 March 1819, Statutes at Large of the United States 3 (1846):493-96; "An Act to Fix the Boundary Line of the Territory of Arkansas, and for Other Purposes," 26 May 1824, Statutes at Large of the United States 4 (1846):40-41; "An Act to Authorize the President of the United States to Run and Mark a Line, Dividing the Territory of Arkansas from the State of Louisiana," 19 May 1828, Statutes at Large of the United States 4 (1846):276-77; "An Act for the Admission of the State of Arkansas into the Union, and to Provide for the Due Execution of the Laws of the United States, Within the Same, and for Other Purposes," 15 June 1836, Statutes at Large of the United States 5 (1856):50-52; Jeannie M. Whayne et al., Arkansas: A Concise History (Fayetteville: University of Arkansas Press, 2019), 81-107; Walter L. Brown, "Arkansas," Dictionary of American History, rev. ed. (New York: Charles Scribner's Sons, 1976), 1:172-73.