Which state sought admission to the Union as a slave state?

The state that sought admission to the Union as a slave state, triggering a substantial conflict, was Missouri. This was during the time of the Missouri Compromise in 1820. The admission of Missouri as a slave state was controversial because it threatened to upset the delicate balance between slave and free states in the Union.

The Missouri Compromise was a significant moment in the history of the United States as it related to the issue of slavery and the balance of power between the North and the South. At the time Missouri wanted to join the Union, there were an equal number of free and slave states. The addition of Missouri as a slave state would tip the balance in favor of slave states in the Senate, where each state had an equal number of seats. To maintain the balance, the Missouri Compromise allowed Missouri to enter the Union as a slave state while Maine was admitted as a free state, maintaining the balance between slave and free states. Furthermore, the Missouri Compromise drew an imaginary line across the former Louisiana Territory, establishing a boundary (36° 30' parallel north) north of which slavery was to be excluded except for Missouri. This compromise, however, only temporarily quelled the tensions between the North and the South over the issue of slavery, which would eventually lead to further conflicts and the American Civil War.