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Why was it necessary to maintain a balance of power between the Slave and Free states?

Because without a balance of Senate seats Congress could vote to make slavery illegal, which they ultimately did while the Civil war was ongoing and all the Southern Senators were absent. The Missouri Compromise kept the balance in tact for decades. When it was renegotiated with the Kansas Nebraska Act days of the South's parity in the Senate were tenuous. Rather than a formula which guaranteed the South's parity as the Missouri Compromised entailed, the Kansas Nebraska act allowed new territories to decide themselves if they would be free or slave. When MissouriKansas voted to come into the union as a free state in 1861 the parity was gone and the Civil War was on.

Sources:

Why was it necessary to maintain a balance of power between the Slave and Free states?

Because without a balance of Senate seats Congress could vote to make slavery illegal, which they ultimately did while the Civil war was ongoing and all the Southern Senators were absent. The Missouri Compromise kept the balance in tact for decades. When it was renegotiated with the Kansas Nebraska Act days of the South's parity in the Senate were tenuous. Rather than a formula which guaranteed the South's parity as the Missouri Compromised entailed, the Kansas Nebraska act allowed new territories to decide themselves if they would be free or slave. When Missouri voted to come into the union as a free state in 1861 the parity was gone and the Civil War was on.

Sources:

Why was it necessary to maintain a balance of power between the Slave and Free states?

Because without a balance of Senate seats Congress could vote to make slavery illegal, which they ultimately did while the Civil war was ongoing and all the Southern Senators were absent. The Missouri Compromise kept the balance in tact for decades. When it was renegotiated with the Kansas Nebraska Act days of the South's parity in the Senate were tenuous. Rather than a formula which guaranteed the South's parity as the Missouri Compromised entailed, the Kansas Nebraska act allowed new territories to decide themselves if they would be free or slave. When Kansas voted to come into the union as a free state in 1861 the parity was gone and the Civil War was on.

Sources:

1
source | link

Why was it necessary to maintain a balance of power between the Slave and Free states?

Because without a balance of Senate seats Congress could vote to make slavery illegal, which they ultimately did while the Civil war was ongoing and all the Southern Senators were absent. The Missouri Compromise kept the balance in tact for decades. When it was renegotiated with the Kansas Nebraska Act days of the South's parity in the Senate were tenuous. Rather than a formula which guaranteed the South's parity as the Missouri Compromised entailed, the Kansas Nebraska act allowed new territories to decide themselves if they would be free or slave. When Missouri voted to come into the union as a free state in 1861 the parity was gone and the Civil War was on.

Sources: