Well we know Abe wanted to ensconce slavery into the Constitution, if the South didn’t secede. We also know he intended to war on the South if they refused to abide by federal laws, like the recently passed Morrill Tariff that was passed without any southern votes. He made all this perfectly clear in his first inaugural speech.
He then set up events at Ft Sumter to invade the South. So, what can we conclude from this? There wouldn’t have been a war had Abe not invaded to impose the tariff. So from the aggressor’s point of view, the war wasn’t about slavery. It was about money.
In fact, those who were hellbent upon perpetuating slavery and extending the odious practice to new territories refused to recognize the legitimately-elected President of all 33 United States, and also rejected the Constitution of the United States.
They bombarded the United States military at Fort Sumter because it was a symbol of the nation, not because such an act of aggression had any strategic value. They thus commenced their hostilities against the United States, and the President deployed the United States military on United States soil to fulfill his oath of office and preserve the union.
They specified
"slavery" in their Declarations of Causes.
A few examples:
GEORGIA "The people of Georgia having dissolved their political connection with the Government of the United States of America, present to their confederates and the world the causes which have led to the separation. For the last ten years we have had numerous and serious causes of complaint against our non-slave-holding confederate States with reference to the subject of African slavery..."
MISSISSIPPI "Our position is thoroughly identified with the institution of slavery-- the greatest material interest of the world. Its labor supplies the product which constitutes by far the largest and most important portions of commerce of the earth. These products are peculiar to the climate verging on the tropical regions, and by an imperious law of nature, none but the black race can bear exposure to the tropical sun. These products have become necessities of the world, and a blow at slavery is a blow at commerce and civilization. That blow has been long aimed at the institution, and was at the point of reaching its consummation..."
SOUTH CAROLINA "... They have encouraged and assisted thousands of our slaves to leave their homes; and those who remain, have been incited by emissaries, books and pictures to servile insurrection... a war must be waged against slavery until it shall cease throughout the United States."