Although Spain fortified St. Louis and a few other outposts during the American Revolution and beat back a British-Indian attack on St. Louis in 1780, the Spanish did not attempt to settle Missouri. However, they did allow Americans to migrate freely into the territory. Spanish authorities granted free land to the new settlers, relaxed their restrictions against Protestants, and welcomed slaveholding families from southern states – especially important after 1787, when slavery was banned in the Northwest Territory. Pioneers such as Daniel Boone arrived from Kentucky, and the Chouteau fur-trading family gained a lucrative monopoly among the Osage. Spanish rule ended abruptly in 1800 when Napoleon forced Spain to return Louisiana to France. Included in the Louisiana Purchase, Missouri then became part of the US in 1803. After the Lewis and Clark expedition (1804-6) had successfully explored the Missouri River, Missouri in general, and St. Louis in particular, became the gateway to the West.

Missouri was part of the Louisiana Territory (with headquarters at St. Louis) until 1 October 1812, when the Missouri Territory (including present-day Arkansas, organized separately in 1819) was established. A flood of settlers between 1810 and 1820 more than tripled Missouri’s population from 19,783 to 66,586, leading Missourians to petition the US Congress for statehood as early as 1818. But Congress, divided over the slavery issue, withheld permission for three years, finally approving statehood for Maine and Missouri under the terms of the Missouri Compromise (1820), which sanctioned slavery in the new state but banned it in the rest of the former Louisiana Territory north of Arkansas. Congress further required that Missouri make no effort to enforce a state constitutional ban on the immigration of free Negroes and mulattos; once the legislature complied, Missouri became the 24th state on 10 August 1821, Alexander McNair became the state’s first governor, and Thomas Hart Benton was one of the state’s first two US senators; Benton remained an important political leader for more than three decades.

Aided by the advent of steamboat travel on the Mississippi and Missouri rivers, settlers continued to arrive in the new state, whose population surpassed 1 million by 1860. The site for a new capital, Jefferson City, was selected in 1821, and five years later the legislature met there for the first time. French fur traders settled the present site of Kansas City in 1821 and established a trading post at St. Joseph in 1827. Mormons came to Independence during the early 1830s but were expelled from the state and crossed the Mississippi back into Illinois. For much of the antebellum period, the state’s economy flourished, with an emphasis on cotton, cattle, minerals (especially lead and zinc), and commerce-notably the outfitting of wagon trains for the Santa Fe and Oregon trails. On the eve of the Civil War, more than half the population consisted of Missouri natives; 15% of the white population was foreign-born, chiefly German and Irish. Black slaves represented only 9% of the total population-the lowest proportion of any slave state except Delaware-while only about 25,000 Missourians were slave holders. Nevertheless, there was a great deal of proslavery sentiment in the state, and thousands of Missourians crossed into neighboring Kansas in the mid-1850s to help elect a proslavery government in that territory. State residents were also active in the guerrilla warfare between proslavery forces and Free Staters that erupted along the border with “bleeding Kansas.” The slavery controversy was exacerbated by the US Supreme Court’s 1857 decision in the case of Dred Scott, a slave formerly owned by a Missourian who had temporarily brought him to what is now Minnesota, where slavery was prohibited; Scott’s suit to obtain his freedom was denied by the Court on the grounds that it was unconstitutional to restrict the property rights of slave holders, in a decision that voided the Missouri Compromise reached 37 years earlier.

During the Civil War, Missouri remained loyal to the Union, though not without difficulty. When the conflict began, Governor Claiborne Fox Jackson called out the state militia “to repel the invasion” of federal forces, but pro-Union leaders such as Francis P. Blair deposed Jackson on 30 July 1861. Missouri supplied some 110,000 soldiers to the Union and 40,000 to the Confederacy. As devastating as the 1,162 battles or skirmishes fought on Missouri soil-more than in any other state except Virginia and Tennessee-was the general lawlessness that prevailed throughout the state; pro-Confederate guerrilla bands led by William Quantrill and Cole Younger, as well as Unionist freebooters, murdered and looted without hindrance. In October 1864, a Confederate army under Major General Sterling Price was defeated at the Battle of Westport, on the outskirts of Kansas City, ending the main military action. Some 27,000 Missourians were killed during the war. At a constitutional convention held in January 1865, Missouri became the first slave state to free all blacks.