Arkansas Territory replaces Missouri Territory
History
The first official use of the name Arkansas came in 1806 when the southern portion of New Madrid County in Louisiana Territory was designated as the District of Arkansas. In 1813, it became Arkansas County in Missouri Territory. When Missouri applied for statehood, it asked for a southern boundary at 36º30′, except for a small portion between the St. Francis River and the Mississippi River where it dropped to 36º. This became the northern boundary of what became Arkansas Territory.On March 2, 1819, at the penultimate meeting of the 15th United States Congress, Congress passed the Arkansas organic act (3 Stat. L. 493), providing for the creation of the Arkansaw Territory on July 4, 1819, from the portion of the Missouri Territory lying south of a point on the Mississippi River at 36 degrees north latitude running west to the St. Francis River, then followed the river to 36 degrees 30 minutes north latitude, then west to the territorial boundary. This included all of the present state of Oklahoma south of the parallel 36°30' north.  The westernmost portion of the territory was removed on November 15, 1824, and a second westernmost portion was removed on May 6, 1828, reducing the territory to the extent of the present state of Arkansas.
Originally the western border of Missouri was intended to go due south to the Red River. During negotiations with the Choctaw in 1820, however, Andrew Jackson unknowingly ceded more of Arkansas Territory. Then in 1824, after further negotiations, the Choctaw agreed to move farther west, but only by "100 paces" of the garrison on Belle Point. This resulted in the bend in the common border at Fort Smith.The territory originally had nine counties: Arkansas, Clark, Crawford (which included Lovely's Purchase), Hempstead, Independence, Lawrence, Miller,  Phillips, and Pulaskị.