well in the northern states, slavery wasnt abundant because it made no sense economically, why spend thousands on slaves when you have tons of irish immigrants that are basically disposable and you pay an extreme fraction of the cost for. The south didnt really have that kind of labor pool.
At the outset of the Civil War Robert E. Lee freed all his slaves, Lincoln had to force Grant to release his slaves at the issue of the Emancipation Proclimation. The Emancipation Proclomation itself didnt free slaves in the north, only in reconqured southern territories. The civil war had been brewing for several years, it should have broken out several years before because of the Nullification Crisis, however Andrew Jackson handled that situation. Is what pushed everything over the top was the election of Lincoln when he wasnt on any Ballots in the south. When that happened the southern states had believed they had lost their political efficacy, and therefore began to secceed.
This is a very short undetailed explanation. I have a 45 page paper on the topic that i wrote in my undergrad.
