Created from what remained of the Louisiana Territory when Louisiana achieved statehood in 1812, the Missouri Territory initially encompassed a large amount of territory west of the Mississippi River. In 1819, Congress reduced it by creating the Arkansas Territory from all of the territory below the 36 30 parallel. It lost its territorial status on August 10, 1821, when Missouri became a state, and Congress gradually reorganized the remaining land into different territories over the succeeding decades. St. Louis was the territorial capital.
Howard L. Conard, ed., Encyclopedia of the History of Missouri (New York: Southern History, 1901), 4:416-18; "An Act for the Admission of the State of Louisiana into the Union, and to Extend the Laws of the United States to the Said State," 8 April 1812, Statutes at Large of the United States 2 (1845):701-4; "An Act Providing for the Government of the Territory of Missouri," 4 June 1812, Statutes at Large of the United States 2 (1845):743-47; "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 Authorize the People of the Missouri Territory to Form a Constitution and State Government, and for the Admission of such State into the Union on an Equal Footing with the Original States, and to Prohibit Slavery in Certain Territories,” 6 March 1820, Statutes at Large of the United States 3 (1846):545-48; "Resolution Providing for the Admission of the State of Missouri into the Union, on a Certain Condition," 2 March 1821, Statutes at Large of the United States 3 (1846):645.