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I didn't realize the significance of that map when I first saw it. It wasn't until later when people informed me that nearly 3/4ths of the earned export value of the USA came from the Southern States. As soon as I realized that the Southern states were making all the money, yet the money was ending up in New York that I realized there was a much bigger picture here than what I was taught when growing up.
I have since filled in additional details. Not only was the North going to lose huge sums of money if the South became independent, but the South would have subsequently built competing industries to the North, and eventually brought all the Midwestern states within it's sphere of influence.
It would have eventually became the larger and more powerful of the two sections of what used to be the USA. The only way the North could stop this was to stop Southern independence, and for that they needed a war.