As I understand it, New Orleans was becoming a much bigger deal by 1860. Steam ships running up and down the Mississippi. Railroads moving east and west to connect to the river. New Orleans sending more goods overseas. New York faced the possibility of getting completely cut out of the deal. The big money up North didn’t like that.
In 1840, New Orleans was the 3rd largest city in the US, after NYC and Baltimore, and the 4th busiest port in the world, after London, Liverpool, and New York.
By 1860, though, things had begun to turn. Construction of canals and then railroads meant that the Midwestern states could send their produce directly to the large East Coast markets rather than to New Orleans.
The Mississippi needed dredging to remain navigable. Ocean-going ships were becoming larger and couldn’t reach the city without dredging at the mouth of the river.
New York had rail connections to a much larger market. Shipping time to Europe was also less. NYC also took more time and trouble to develop the financial structures that international trade demanded.
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Yes. I've read newspaper articles and various commentary from that time period, and they absolutely did not like the idea of New Orleans not being under their control.
Allowing the South to secede peacefully would have been a financial disaster for the North. It also would have effectively repealed Union tariffs because of the wide porous borders between the Confederacy and the Union, and the fact the Mississippi reaches so deeply into Northern states.
Secession would have undermined their Federal income schemes and disabled most of their protectionist laws.
It would have cost them big money in the North.
It is no accident that they concentrated so much effort on taking Vicksburg and New Orleans in the war.