In the lead-up to the Civil War, America consisted of just 34 states. And of those, seven of the southern states were adamant that they would not follow the northern states in abolishing slavery. The first state to bring an end to slavery was Pennsylvania, which did so as early as 1780. Many other northern states then followed suit over the coming decades.
But for the South, slavery was strongly connected with plantation agriculture, which was the main economic engine of the region. In 1860 the slave population in America had in fact reached four million. And most of those slaves were in the southern states.
The southern states argued that the Union was just an agreement and that it could not be enforced. In other words, went the argument, any state could choose to leave the Union at will. But the northern states saw things quite differently. As far as they were concerned, the Founding Fathers had intended that the Union should be a binding and permanent institution. Membership was not voluntary, they said.