FOLLOW-UP ON "NOT A WAR TO ABOLISH SLAVERY"
Of course, another question that could be asked about the argument that the war was fought over slavery is:
If the war was fought over slavery, why did Lincoln exempt from the Emancipation Proclamation all Northern slaves and all Southern slaves who lived in areas in the South under federal control?
Nowhere will one find a statement by Lincoln or any other Republican to the effect that if the South would end slavery the North would withdraw its troops and/or let the South leave in peace. The number one issue, the key sticking point, was Southern independence, not slavery.
If you want to discover the issue over which any war is fought, just look at the conditions that each side gives as its requirement to end the war. The North's only condition for ending the war was the repudiation of Southern independence. If the South had abolished slavery during the war, the North would not have halted its invasion. Never at any time did any Northern leader say, "If the South will end slavery, we will withdraw our armies and allow the South to leave in peace."
The South's only condition for ceasing to defend itself was the withdrawal of Northern armies. If the North had let the South leave in peace in the first place, there would have been no war to begin with. If the North had decided at any point during the war to withdraw its armies, the fighting would have stopped. But if the South had ended slavery during the war, the fighting would not have stopped.
There were plenty of Southerners, probably a fairly solid majority, who were more than willing to end slavery. Most Confederate soldiers did not own slaves. Even a majority was Confederate generals did not own slaves. In fact, some of the strongest opponents of secession were slaveholders, especially Whig slaveholders.
Civil War website