If the issue of Slavery was the cause of the civil war - then why did the North continue to allow slavery?
"On New Year's Day, 1863, Lincoln issued the final Emancipation Proclamation. Contrary to what its title suggests, however, the presidential edict did not immediately free a single slave. It "freed" only slaves who were under Confederate control, and explicitly exempted slaves in Union-controlled territories, including federal-occupied areas of the Confederacy, West Virginia, and the four slave-holding states that remained in the Union.
The Proclamation, Secretary Seward wryly commented, emancipated slaves where it could not reach them, and left them in bondage where it could have set them free. Moreover, because it was issued as a war measure, the Proclamation's long-term validity was uncertain. Apparently any future President could simply revoke it. "The popular picture of Lincoln using a stroke of the pen to lift the shackles from the limbs of four million slaves is ludicrously false," historian Allan Nevins has noted."
http://www.ihr.org/jhr/v13/v13n5p-4_Morgan.html
Slaves were owned in the North during the civil war - and in fact the "hero" of the Civil War - Ulysses S. Grant - Union Military leader at the end of the war as well as President after Lincoln and Jackson owned slaves.
The fact is that slavery WAS an issue - but it was not by any means the only one - or even the largest one. It has been portrayed as such because morally - it is an abhorrent practice, and the victor gets to write the history. What better causus belli for later generations to look at than a vile acceptance of such practices?