By 1860, New Orleans had become the forth largest tonnage port in the country. The port city served as the staging and organizational point for the flow of wheat, flour, furs, corn, tobacco, minerals, and cotton flowing from Southern and Midwestern states and territories to the European markets.
A way to measure the value of the New Orleans port to the Union is that it was where the commodities of Midwestern and Southern agriculture went out to the world and the bulk commodities of industrialism came in. The commodity chain of the global cotton and European food industry largely started there, as did that of Southern industrialism. If these facilities were gone from the Union, more than the price of goods shifted: The very physical structure of the Union economy would have to be reshaped.
Port city interests in New York, Baltimore, Boston knew that if the Mississippi River was shut to Midwestern traffic resulting from secession, then the foundations of the Union economy would be shattered. The industrial minerals needed in the factories would not come in, and the agricultural wealth would not flow out under Federal control.
Compared to overland shipping, river transport was cheap, and most of the agricultural products had low value-to-weight ratios. The U.S. transport system serving the Midwest of the 1860s was built on the assumption that these commodities would travel to and from New Orleans by barge, where they would be loaded on ships or offloaded. If Louisiana left the Union, there were not enough wagons or rail cars to handle the long-distance hauling of these enormous quantities — assuming that the economics could be managed, which they could not be.
The United States historically depended on the Mississippi and its tributaries for transport. Barges navigate the river. Ships go on the ocean. The barges must offload to the ships and vice versa. There must be a facility to empower this exchange. It is also the facility where goods are stored in transit. Without this port, the river could not be used.
Protecting that port has been, from the time of the Louisiana Purchase, a fundamental national security issue for the United States.
Shipping interests from the North reminded Lincoln of this in their meetings with him prior to his orders to Gustavus Fox to initiate the expeditions to Charleston and Pensacola.
You are no doubt well aware of how utterly the Union economy disintegrated between April, 1861 and July, 1863 when the Mississippi River was unavailable for shipping.
The industrial minerals needed in the factories would not come in
I'm curious. What raw materials of industry, mineral or otherwise, did the USA import through New Orleans?
But thanks for recognizing that shutting off a nation from its access to the sea has historically been recognized as a casus belli.
Lots of other pro-Confederates try to claim the South would never have tried to use its possession of the mouth of the Mississippi to interfere with the trade of the Midwest. In actual fact it most certainly would have. Had the CSA been allowed to secede peacefully, what do you think would have happened the first time the return of a fugitive slave was refused? Close the Mississippi, of course