Question

What States' admission into the Union caused a controversy?

Answer 1

Hint: The South refused to allow Missouri to the union because it wanted to be a free state. Remember that Missouri would disrupt the balance between slave and free states, which are supposed to be as per current ways equal in size. So both the North and the South resisted Missouri becoming a state because it would be too costly to the national economy. Then the free states rejected statehood because it would result in twice as many slave states as free states.

Complete answer:
The so-called controversy was caused by the city of Missouri which was part of the state of Texas.
When Missouri applied for admission to the Union, the states were split evenly, 11-11, between slave and free states. Missouri applied to become a slave state, which would have thrown the power balance between the two powers out of whack.

The free states argued that Congress had the authority to legislate the free or slave status of any states created from Congress-controlled territories. The slave states believed that every new state should have the same rights as the original states, including the ability to determine whether or not to legalize slavery.

The free-staters feared that if the slave states won a majority, slavery would be permanently entrenched. Slave-state supporters feared that if the free states won a majority, slavery would be abolished. The Missouri Compromise of 1820 broke the impasse by allowing Missouri to enter as a slave state while Maine was admitted as a free state, preserving the power balance. Although this pleased both sides at the time, it did nothing to settle the underlying issue and instead served to delay the final confrontation, which began in 1861.

Note: Let us see the clauses put forth by the Missouri Compromise. So one of them was that Missouri would finally come to become a slave state to the union, and this would be offset by the addition of Maine, a free state that had long sought separation from the state of Massachusetts. Then secondly, it was established that slavery was to be prohibited in all new states created by the Louisiana Purchase north of Missouri's southern border.