Claimed by France as part of the Louisiana Territory in 1682, Missouri had only a few small river towns by the start of the 19th century when the land passed to American hands and Lewis and Clark pushed up the Missouri River. Missouri was admitted to the Union as a slave state in 1821, per the Missouri Compromise (which permitted slavery in Missouri but prohibited it in any other part of the Louisiana Territory above the 36°30´ parallel), but abolitionists never compromised their ideals and bitter feelings were stoked along the Missouri–Kansas border by Civil War time.

The state’s ‘Show-Me’ nickname is attributed to Congressman Willard Duncan Vandiver, who said in an 1899 speech, ‘I come from a state that raises corn and cotton and cockleburs and Democrats, and frothy eloquence neither convinces nor satisfies me. I am from Missouri. You have got to show me.’ The name now implies a stalwart, not-easily-impressed character.

Since the brief period of radical Republican rule from 1864 to 1870, Missouri has been permanently wedded to neither major party. While tending toward the Republicans in the days of Theodore Roosevelt, it turned solidly Democratic for Franklin D. Roosevelt and helped to elect Missourian Harry S. Truman to the presidency in 1948. Political machines in the large cities have attracted national attention, notably the machine of Thomas J. Pendergast (1872–1945) in Kansas City. Missouri has contributed to the United States such outstanding statesmen as Champ Clark, James Reed, and W. Stuart Symington. Thomas Hart Benton, a descendant of the Missouri Senator of the same name, was one of the country’s important artists.