It was fought for economic control of the South. The South accounted for 3/4ths of all European trade with the United States. Had the South became independent, Almost all of the Trade with Europe would have moved to the Southern States leaving New York with drastically reduced business.
Here is an example of what European Trade looked like before the Civil War. (and Remember, 3/4ths of that money pile on New York was earned by Southern products.)
Worse still for the North was the fact that all the extra profit and capital that would be created by Independent trade with Europe would have produced new Industries in the South that would have competed head to head with Northern Industries, further weakening the North economically.
Over time, all the Midwestern states would have been supplied from businesses in the South through usage of the Mississippi river, and the New York/Chicago industries would have lost heavily in those markets too.
The Union went to war with the South because an independent South was a grave financial threat to the economic interests of the North. Corporate Lawyer Abraham Lincoln and his Wealthy New York backers fully realized this, and that is why a war was absolutely necessary.
Of course the very first thing Lincoln did was to throw up an economic blockade around the South, because if Low duty trade with Europe had ever gotten started, there would be no way to put that genie back into the bottle.
The Civil war was a war for economic control of the South. That is why Lincoln was still willing to let them keep slavery in his letter to Horace Greeley in August of 1862.
What he wasn't willing to let them do is trade goods and services with Europe unless Northern economic interests could take a cut.
That the war was about "Slavery" was just propaganda to hide the fact that he had just gotten 750,000 people killed to maintain economic control of the South.