Question

when Thomas Jefferson heard that Missouri was barred from the union at first because of its slave status, he "considered it at once as the knell of the union" why?

Answer 1

to preserve the balance of power in Congress between slave and free states, the Missouri Compromise was passed in 1820 admitting Missouri as a slave state and Maine as a free state. Furthermore, with the exception of Missouri, this law prohibited slavery in the Louisiana Territory north of the 36° 30´ latitude line. In 1854, the Missouri Compromise was repealed by the Kansas-Nebraska Act. Three years later the Missouri Compromise was declared unconstitutional by the Supreme Court in the Dred Scott decision, which ruled that Congress did not have the authority to prohibit slavery in the territories.