Slavery was a slow march to civil war.  As the U.S. acquired more territory, whether slavery would be expanded became a bitter dispute for southerners who feared these new territories would not allow it and that abolitionists would take over the government.  It was banned in states above the mason-dixon line.  Southerners resented the banning of slave-holding in these new states (they would have to be freed if they did) while northern industrialists were busy establishing factories.  They were also p’d b/c of what they saw as increasing hostility toward slavery, northern states wouldnt return escaped slaves nor allow transit through their states - slave owners liked spending summers in the Hamptons and could no longer bring their slaves through NY.  When Lincoln who ran an anti-slavery campaign won, southern states immediately began to secede.  BTW, Abe went to war purely to keep the country together not to free slaves, and said as much in his 1862 letter to NY tribune.