The Platte Purchase was a land acquisition in 1836 by the United States government from American Indian tribes all of which was east bank lands along the Missouri River that added 3,149 square miles (8,156 km2) to the northwest corner of the state of Missouri. This expansion of the slave state of Missouri was done in violation of the Missouri Compromise of 1820, which prohibited the extension of slavery in the former Louisiana Territory north of the parallel 36°30′ north except within the boundaries of the state of Missouri as defined at the adoption of the Missouri Compromise. The area acquired is almost as large as the states of Delaware and Rhode Island combined, and extended Missouri north and westward along the river. St. Joseph, one of the main ports of departure for the westward migration of American pioneers, was located in the new acquisition.
The region includes the following modern counties within its bounds: Andrew (435 square miles, 1127 km²), Atchison (545 square miles, 1412 km²), Buchanan (410 square miles, 1062 km²), Holt (462 square miles, 1197 km²), Nodaway (877 square miles, 2271 km²), and Platte (420 square miles, 1088 km²). It also includes the northwest suburbs of Kansas City, a small area of Kansas City proper, the cities of St. Joseph, Mound City and Maryville, Missouri, as well as Kansas City International Airport and almost all of Missouri's portion of Interstate 29, save the small portion which runs concurrently with Interstate 35 in Clay County.
American Indian people living in unorganized portions of the Missouri Territory had been promised that they would keep their land permanently. European Americans began to encroach on the territory, violating the agreement with the United States government. Joseph Robidoux lobbied for incorporation of settled territory, located east of the Missouri River to the latitude of the state's northern border, into the recently formed state of Missouri.
An agreement was reached in 1836 with the chiefs Mahaska and No Heart of the Ioway tribe and leaders of the combined Sac and Fox tribes in a ceremony at Fort Leavenworth, Kansas that was presided by William Clark, then the Superintendent of Indian Affairs and based in St. Louis. (He was one of the leaders of the Lewis and Clark Expedition.)
The tribes were paid $7,500 for their land. The U.S. government was to "build five comfortable houses for each tribe, break up 200 acres (0.8 km2) of land, fence 200 acres (0.8 km2) of land, furnish a farmer, blacksmith, teacher, interpreter, provide agricultural implements, furnish livestock", and a host of other small items. The tribes agreed to move to reservations west of the Missouri River in what was to become Kansas and Nebraska, today known as the Ioway Reservation and the Sac and Fox Reservation.
The western border of Missouri was established at the mouth of the Kaw River in Kansas City (94 degrees, 36 minutes west longitude) (which is also the border between Missouri and Kansas). The purchase extended the Missouri border in the northwest to 95 degrees, 46 minutes west longitude.
The annexation was approved by the Missouri Legislature on 21 November 1836, and with it the state briefly became the largest in the Union by geographic size. On 28 March 1837, President Martin Van Buren issued a proclamation supporting the annexation.
Current maps show the eastern border of Platte County and all counties north further east than the border between Missouri and Kansas south of the river, which no longer conforms to the mouth of the Kaw River.
- Hilde Heun Kagan, Ed. in Charge, The American Heritage Pictorial Atlas of United States History, New York: American Heritage Publishing Co., Inc., 1966, p. 199.
- Roger D. Launius, Alexander William Doniphan: Portrait of a Missouri Moderate, Columbia: University of Missouri Press, 1997, p. 31.
- By this time, however, Michigan had been admitted to the Union, making Missouri second in size. Virginia, which included West Virginia until Dec. 31, 1862, was slightly larger than pre-annexation Missouri.