The reason the UNION went to war with the South had nothing to do with slavery, at least not slavery regarding black people. Why did the Union invade?
Tariffs and taxes are not mentioned in the declaration.
Those declarations were only a few in Number, Slavery is only prominent in a few of them, and of course you have left out the ones that do mention tariff's and taxes because that does not suit your preference.
This one quite clearly says that the economic destruction perpetuated on the South by the North is the primary cause of their discontent with the existing government.
http://www.civilwarcauses.org/rhett.htm
The United States government sent troops to suppress a rebellion. A rebellion caused by several southern states not liking the outcome of an election. These states sent envoys to other states to attempt to get them to join the rebellion. They also captured federal arsenals and other federal properties, before the new president had even been inaugurated. They then fired upon US ships and their rebellion declared war against the US.
Now during the course of this conflict the US government also adopted the goal of ending slavery as a war aim.