It was the called the Confederate States of America, and the Republicans killed 600,000 Americans to stop it.
It would have completely upended their tariff plans, as noted by William Sherman in a letter to his wife, January 20, 1861:
Down here they think they are going to have fine times. New Orleans a free port whereby she can import goods without limit or duties and sell to the up-river countries. But Boston, New York, Philadelphia, and Baltimore will never consent that New Orleans should be a free port and they subject to duties. The most probable result will be that New Orleans will be shut off from all trade, and the South having no money and no sailors cannot raise a blockade without assistance from England, and that she will never receive.
I have letters from General Graham and others who have given up all hope of stemming the tide. All they now hope for is as peaceable a secession as can be effected. I heard Mr. Clay’s speech in 1850 on the subject of secession and if he deemed a peaceable secession then as an absurd impossibility, much more so is it now when the commercial interests of the North are so much more influential. . .