Another stage of U.S. expansion took place when inhabitants of Missouri began petitioning for statehood beginning in 1817. The Missouri territory had been part of the Louisiana Purchase and was the first part of that vast acquisition to apply for statehood. By 1818, tens of thousands of settlers had flocked to Missouri, including slaveholders who brought with them some ten thousand slaves. When the status of the Missouri territory was taken up in earnest in the U.S. House of Representatives in early 1819, its admission to the Union proved to be no easy matter, since it brought to the surface a violent debate over whether slavery would be allowed in the new state.

Politicians had sought to avoid the issue of slavery ever since the 1787 Constitutional Convention arrived at an uneasy compromise in the form of the “three-fifths clause.” This provision stated that the entirety of a state’s free population and 60 percent of its enslaved population would be counted in establishing the number of that state’s members in the House of Representatives and the size of its federal tax bill. Although slavery existed in several northern states at the time, the compromise had angered many northern politicians because, they argued, the “extra” population of slaves would give southern states more votes than they deserved in both the House and the Electoral College. Admitting Missouri as a slave state also threatened the tenuous balance between free and slave states in the Senate by giving slave states a two-vote advantage.

The debate about representation shifted to the morality of slavery itself when New York representative James Tallmadge, an opponent of slavery, attempted to amend the statehood bill in the House of Representatives. Tallmadge proposed that Missouri be admitted as a free state, that no more slaves be allowed to enter Missouri after it achieved statehood, and that all enslaved children born there after its admission be freed at age twenty-five. The amendment shifted the terms of debate by presenting slavery as an evil to be stopped.

Northern representatives supported the Tallmadge Amendment, denouncing slavery as immoral and opposed to the nation’s founding principles of equality and liberty. Southerners in Congress rejected the amendment as an attempt to gradually abolish slavery—not just in Missouri but throughout the Union—by violating the property rights of slaveholders and their freedom to take their property wherever they wished. Slavery’s apologists, who had long argued that slavery was a necessary evil, now began to perpetuate the idea that slavery was a positive good for the United States. They asserted that it generated wealth and left white men free to exercise their true talents instead of toiling in the soil, as the descendants of Africans were better suited to do. Slaves were cared for, supporters argued, and were better off exposed to the teachings of Christianity as slaves than living as free heathens in uncivilized Africa. Above all, the United States had a destiny, they argued, to create an empire of slavery throughout the Americas. These proslavery arguments were to be made repeatedly and forcefully as expansion to the West proceeded.

Most disturbing for the unity of the young nation, however, was that debaters divided along sectional lines, not party lines. With only a few exceptions, northerners supported the Tallmadge Amendment regardless of party affiliation, and southerners opposed it despite having party differences on other matters. It did not pass, and the crisis over Missouri led to strident calls of disunion and threats of civil war.

Congress finally came to an agreement, called the Missouri Compromise, in 1820. Missouri and Maine (which had been part of Massachusetts) would enter the Union at the same time, Maine as a free state, Missouri as a slave state. The Tallmadge Amendment was narrowly rejected, the balance between free and slave states was maintained in the Senate, and southerners did not have to fear that Missouri slaveholders would be deprived of their human property. To prevent similar conflicts each time a territory applied for statehood, a line coinciding with the southern border of Missouri (at latitude 36° 30′) was drawn across the remainder of the Louisiana Territory. Slavery could exist south of this line but was forbidden north of it, with the obvious exception of Missouri.

The Missouri Compromise resulted in the District of Maine, which had originally been settled in 1607 by the Plymouth Company and was a part of Massachusetts, being admitted to the Union as a free state and Missouri being admitted as a slave state.