Missouri was part of the 1803 Louisiana Purchase by the United States, and earned the nickname "Gateway to the West" because it served as a major departure point for expeditions and settlers heading to the West in the 19th century. St. Charles, just west of St. Louis, was the starting point and the return destination of the Lewis and Clark Expedition, which departed up the Missouri River in 1804 to explore the western territories to the Pacific Ocean. St. Louis was a major supply point for decades for parties of settlers heading west.
As many of the early American settlers in western Missouri migrated from the Upper South, they...settled predominantly in 17 counties along the Missouri River, in an area of flatlands that enabled plantation agriculture and became known as "Little Dixie". In 1821 the territory was admitted as a slave state as part of the Missouri Compromise with a temporary state capitol in St. Charles. In 1826 the capital was shifted to its permanent location of Jefferson City, also on the Missouri River.