That is an outrageous lie on your part. The Union had FOUR slave states of its own throughout the 4 years of the war. KY MD DE and MO were slave states with a total of 300,000 black slaves and Lincoln let them stay in the Union!!! How can the war be about slavery when both sides practiced slavery???
The press has been lying about this for 150 years. The truth is slavery was a small part of the Civil War.
In fact, when Lincoln issued the Emancipation Proclamation, it specifically named the States, or portions thereof, to which the Proclamation applied. NONE of those listed include the four UNION States that had slaves. So much for Lincoln supposed concern for the slaves. He was willing to free them in areas that didn't recognize his authority but did nothing to free them in areas where he actually did.