John Stuart Mill, 1861.
"The Northern onslaught upon slavery was no more than a piece of specious humbug designed to conceal its desire for economic control of the Southern states."
Charles Dickens, 1862.
I know, they don't know what they're talking about. Slavery was the ONLY cause of a war fought by brother against brother. Never mind that the Constitution provided for slavery. Never mind that the Emancipation Proclomation freed no slave. Never mind that Lincoln was not a great proponent of the black race.
"If I could save the Union without freeing any slave I would do it, and if I could save it be freeing all the slaves I would do it; and if I could save it by freeing some an leaving others alone I would do that. What I do about slavery, and the colored race, I do because I believe it helps to save the Union, and what I forbear, I forbear because I do not believe it would help to save the Union."
Abraham Lincoln to Horace Greeley, 22 August 1862.
"It must be admitted, truth compels me to admit...Abraham Lincoln was not, in the fullest sense of the word, either our man or our model. In his interests, in his associations, in his habits of thought, and in his prejudices, he was a white man. He was preeminently the white man's president, entirely devoted to the welfare of white men. He was ready and willing at any time during the last years of his administration to deny, postpone, and sacrifice the rights of humanity in the colored people, to promote the welfare of the white people of his country."
Frederick Douglass, noted African-American leader.
And you're right, slavery wasn't the only reason for the war. But the rift that led to secession had its roots in the preservation of slavery in the states and expansion into the territories (at least so as to avoid a predominance of free states).
Lincoln or no Lincoln, the peculiar institution would have ended sooner or later, and given the predisposition of those who believed that their lifestyle depended upon slave labor not to give it up, the issue was certain to be forced by violence. Just MHO, of course.