In his First Inaugural address, The Lincoln said:
I understand a proposed amendment to the Constitution . . . has passed Congress, to the effect that the Federal Government shall never interfere with the domestic institutions of the States, including that of persons held to service. To avoid misconstruction of what I have said, I depart from my purpose, not to speak of particular amendments, so far as to say that, holding such a provision to now be implied constitutional law, I have no objection to its being made express and irrevocable.
In a letter of August 22, 1862 to Horace Greeley, The Lincoln explained the purpose of the war:
My paramount object in this struggle is to save the Union, and it is not either to save or destroy slavery. If I could save the Union without freeing any slave, I would do it; and if I could save it by freeing some and leaving others alone I would also do that. What I do about slavery, and the colored race, I do because I believe it helps to save the Union.
On July 22, 1861, in a Joint Resolution, the US Congress explained what the war was about:
Resolved: . . . That this war is not being prosecuted upon our part in any spirit of oppression, nor for any purpose of conquest or subjugation, nor purpose of overthrowing or interfering with the rights or established institutions of those states, but to defend and maintain the supremacy of the Constitution and all laws made in pursuance thereof and to preserve the Union, with all the dignity, equality and rights of the several states unimpaired; and that as soon as these objects are accomplished the war ought to cease.
It doesn't matter what Lincoln said, it was the South that seceded and they said it was for slavery.