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NARRATOR: In 1819, representation in the Senate was equally balanced between states that allowed slavery and states that prohibited it. But as territories that had been acquired as part of the Louisiana Purchase began to apply for statehood, one question came up. Would they join with the slave states or the free states?

Missouri's application for admission to the Union threatened to tip the delicate balance in the Senate and sparked the first of many national debates about slavery. Northern politicians immediately called for the Missouri Territory to free its slaves as a condition for entering the Union.

MATTHEW WARSHAUER: And so you get this major divide between the North and the South-- the biggest one that had ever occurred since the creation of the Constitution where they had settled certain issues over slavery. Well, the Missouri Compromise brings it all back up.

NARRATOR: When James Tallmadge, the New York representative, put forth an amendment suggesting gradual emancipation for Missouri slaves, Southerners were outraged. Not only did Southerners in Congress want Missouri to join their ranks as a slave holding state, they didn't want the federal government deciding what a state should do regarding slavery.

The nation's first major debate on slavery had begun. In the North, religious reformers organized anti-slavery rallies bringing their cause to the forefront of national politics. And in the South, the word, "secession" was uttered for the first time.

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But almost as soon as it had begun, the conflict ended with the Missouri Compromise.

MATTHEW WARSHAUER: Henry Clay, who is Speaker of the House at the time, gets most of the credit for that. Because of this compromise and a couple others, he becomes known as, "The Great Compromiser."

NARRATOR: Clay guided a bill through the Senate declaring that Missouri would come into the Union as a slave state, but the northern state of Maine would also be admitted as a free state, maintaining the tenuous balance in the Senate.

As part of the Missouri Compromise, Congress took the line along the southern boundary of Missouri and extended it west across the remainder of the territories in the Louisiana Purchase. Then, Congress declared that slavery would be permitted south of that line, but banned north of it.

The slavery issue had been settled for the moment. But many, like ex-president Thomas Jefferson, predicted that the Missouri debate signaled a crisis that would eventually tear the country apart. "This momentous question, like a fire bell in the night, awakened and filled me with terror," he wrote to a friend. "I considered it, at once, as the knell of the Union."

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