We hear it over and over, the War was to "Preserve the Union." Let's take a look at that.
When the seven states of the deep south issued their bills of secession what happened?
Did the federal government grind to a halt?
Did the US Navy suddenly sink to the seafloor?
Did the rail roads stop running?
Did the posts stop moving?
Was commerce to and from NY, PA, OH, New England, et al disrupted?
No to all of those.
Hmmm....maybe it was minerals.
Iron ore? Lots of that in the north
Coal? I seem to recall that there is still lots of coal in PA and VA (remember, VA was one of the last states to leave the Union, and only after Lincoln pushed to use VA troops to invade the deep south).
No petroleum industry, or only the bare start of it, in PA.
Cotton would still have been sold to northern mills, although northern industrialists would have to pay tariffs on it, thereby cutting into their huge profits.
So, since the government kept working, the relationships among the states in the Union still working as they always had, no loss of minerals or other raw materials, no loss of trade, ships - including slavers - sailing from northern ports unhindered, and foreign trade flooding into northern ports unobstructed, what was so important that the federal government would wage a war of aggression to force seven states back into what had been a voluntary union?
Don't do the fifth grade textbook "SLAVERY!" finger pointing and bleating about abolition - the Corwin Amendment had been passed, and that without the votes in Congress of the seven states that had left the Union, and just awaited ratification, which was a lock if the deep south ratified it. It guaranteed perpetual slavery, and Lincoln had promised to support it. So abolition is out as a reason for the north to wage war.
That leaves only one reason for the federal government to wage war on the south - revenue. The only significant change was flow of gold into the federal treasury. Almost all federal revenue came from tariffs, which is one of the reasons members of Congress from the north kept raising them. And the South paid between 70% and 80% of all tariffs.