Lincoln won the 1860 election on a ticket of no new slave-states.
This meant that the South would always be outvoted in Congress, which would tend to pass laws that favoured the North at the expense of the South.
So the South decided to break away and form a separate nation, taking the huge cotton revenues with them.
Reuniting the states, and retrieving the cotton revenues, was the chief incentive for the North to go to war.
It did not become an Abolitionist war until January 1863.
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15 slave states and 19 free states during the American Civil War
If the state was brought into the union it would upset the balance of free and slave states.
The Potomac divided the Confederacy from the Union, although Maryland was a slave-state, and so was DC at the beginning. The Ohio River divided the slave-states from free soil.
Northerners opposed Missouri's entrance as a slave state because slave states would have more senators in Congress than the free states would. It would give the South a bigger voting presence in Congress. This issue was the basic cause of the Civil War.
Fugitive slave act in 1850 along with government control toward slave states and slave free states.