Slavery was legal in the Union for "four score and seven years" (actually a couple of years longer than that) so why did it just cause a war in 1861?
I also note that the Union didn't go to war with Maryland, Delaware, Kentucky, Missouri, or Tennessee, which were Northern slave states. If they were fighting to end slavery, why didn't they start with their own states?
Lincoln launched the war to prevent the South from trading directly with Europe. If normalized European trade with the South had been allowed to develop, it would have wrecked industries in the North East, and it would have severely hurt Washington D.C.'s money flow.
They fought over the money, not whether or not slavery should be abolished. That was something they came up with midway through the war, and then they kept selling it after the fact to justify what they had done.
The cause of the war was the states seceding! The states seceded because of the election of a Republican to the presidency. Why was a Republican candidate so repugnant to the southern states? Because their platform called for the restriction of slavery from the territories. The southern states had threatened to secede in the election of 1856 if the Republican candidate was elected. Come on man, you cannot be this dense.