That is not what Lincoln said at his first inauguration - or in his call for 75,000 volunteers to attack the South. But as others have suggested, Lincoln's claim that he was fighting to collect taxes due, or to save the union, could have been a pretext for war.
For the sake of this post let's stipulate you are correct: Lincoln and the North were “fighting to free the slaves.”
Let's set aside for a moment no slave owners were attacked by Union forces to "free the slaves" in Delaware, Maryland, Kentucky or other Union slave states.
Slavery was constitutional in the United States because it was enshrined into the United States Constitution. At the founding the states ensured that - although only 13 of the original 13 slave states voted to include it into the constitution.
If Lincoln was “fighting to free the slaves” then he was fighting to overthrow the U.S. Constitution. As president, Lincoln was not supposed to take up arms and levy war against the states for the purpose of violently overthrowing the pro-slavery constitution.
What Lincoln could have done was to offer a constitutional amendment to peacefully and legally abolish slavery. It could have been done without firing a shot. Lincoln and the North could have proposed such an amendment decades earlier if they wanted; and they probably would have if it had been in their own economic and political best self-interest.
You are eager to claim Lincoln took up arms against the U.S. Constitution. Maybe he did.
Lincoln personally abhorred slavery but felt he did not have the constitutional authority as it pertained to war measures and he felt he would lose the loyalty and support of those in the border states were it was still legal. Secession and the South's opening fire on Ft. Sumter decided the issue. The South went to war to preserve slavery so stop the bs, Copperhead.
If Lincoln was “fighting to free the slaves” then he was fighting to overthrow the U.S. Constitution.
Lincoln never claimed the power to free slaves other than as a war power as Commander-in Chief of the army and navy, and then only when absolutely necessary. As late as May 1862, Lincoln counter-mandered miliary proclamations freeing slaves taken by the Union army, rendering such proclamations null and void. Had the Union won the war in 1861, or in early 1862, this documents the lack of authority on the part of the Union to abolish slavery in the South.
The war to abolish slavery was apparently fought for over a year while acknowledging there was no existing lawful authority to abolish slavery, even using military power under martial law.
It is notorious that the initial reason fiven for the war was to preserve the Union. It was not until that fell from favor, and a draft was to become necessary, that the justification changed to a war to abolish slavery.
It is logical nonsense to purport that a Union war to abolish slavery, after four years, had not abolished slavery in the Union states.
It is perfectly explained by the fact that there was no legal authority to end slavery in the Union states other than by constitutional amendment. Slaves in the Confederate states were declared free under a wartime military power under martial law and as a military necessity. That could not be done in the Union states as the Union was not at war with itself.
The War to Abolish Slavery was a marketing campaign, akin to the War to Abolish WMD in Iraq.