if the North allowed the South to secede, the Federal Government would have gone bankrupt and the North would have collapsed economically. “There would have been grass growing in the streets of New York and Boston.”
At that time, the South provided about 60-70% of of the Federal Government’s revenue through duties and tariffs (no income tax, remember?). The South would have controlled the mouth of the Mississippi through which most of the commerce of the West and Midwest flowed. The South planned to levy a 10% tariff in its ports (tariffs were the largest part of the reason the South seceded...), greatly undercutting the North’s tariffs.
That, more than any other reason, was the reason for the Civil War.
The Declarations say it was slavery.