Slavery was very much one of the reasons the South went to war. It was never one of the reasons the North went to war. Lincoln wanted his union and was willing to do anything to preserve it including fighting a war against his countrymen and guaranteeing slavery permanently.
It was those Southern countrymen who chose war in the first place.
Abraham Lincoln was the abolitionist's candidate for president. In his June 16, 1858 speech, he strongly argued that a nation could not exist "half slave and half free." Then in July 10, 1858, he addressed an audience at the Tremont Hotel in Chicago.
...but when they look through that old Declaration of Independence they find that those old men say that "We hold these truths to be self-evident, that all men are created equal," and then they feel that that moral sentiment taught in that day evidences their relation to those men, that it is the father of all moral principle in them, and that they have a right to claim it as though they were blood of the blood, and flesh of the flesh of the men who wrote that Declaration, and so they are. That is the electric cord in that Declaration that links the hearts of patriotic and liberty-loving men together, that will link those patriotic hearts as long as the love of freedom exists in the minds of men throughout the world.These are not the words and sentiments of a man that is prepared to preserve slavery permanantly. These are the words and sentiments of an American giant whose shoes you would not be fit to shine.
Now, sirs, for the purpose of squaring things with this idea of "don't care if slavery is voted up or voted down" [Douglas's "popular sovereignty" position on the extension of slavery to the territories], for sustaining the Dred Scott decision, for holding that the Declaration of Independence did not mean anything at all, we have Judge Douglas giving his exposition of what the Declaration of Independence means, and we have him saying that the people of America are equal to the people of England. According to his construction, you Germans are not connected with it.
...What are these arguments? They are the arguments that kings have made for enslaving the people in all ages of the world. You will find that all the arguments in favor of king-craft were of this class; they always bestrode the necks of the people, not that they wanted to do it, but because the people were better off for being ridden. That is their argument, and this argument of the Judge is the same old serpent that says you work and I eat, you toil and I will enjoy the fruits of it. Turn in whatever way you will---whether it come from the mouth of a King, an excuse for enslaving the people of his country, or from the mouth of men of one race as a reason for enslaving the men of another race, it is all the same old serpent, and I hold if that course of argumentation that is made for the purpose of convincing the public mind that we should not care about this, should be granted, it does not stop with the negro. I should like to know if taking this old Declaration of Independence, which declares that all men are equal upon principle and making exceptions to it where will it stop. If one man says it does not mean a negro, why not another say it does not mean some other man? If that declaration is not the truth, let us get the Statute book, in which we find it and tear it out! Who is so bold as to do it!