When the Constitution was first being debated, George Mason of Virginia introduced a bill to prevent the passage of any tariff that did not receive a two-thirds majority. He warned, "The effect of a provision to pass commercial laws by a simple majority would be to deliver the south bound hand and foot to the eastern states". His words proved so true as early as the 1828 Tariff of Abominations, and even more so after the events of 1860.
What a lot of people do not realize is that the Republican platform not only called for stopping the introduction of slavery to new territories, it also called for the introduction of new, higher tariffs. In March of 1861, Congress passed and Buchanan signed into law the Morill tariff. This tariff imposed the highest tariffs in US history, with over a 50% duty on iron products and 25% on clothing; rates averaged 47%. This threatened the bankruptcy of the entire South. The deep southern states of South Carolina, Alabama, Louisiana, Mississippi, Florida, and Georgia had already rebelled and formed the Confederacy at this time, in anticipation of the Republican tariffs. As one Texas delegate to Congress stated, "You are not content with the vast millions of tribute we pay you annually under the operation of our revenue law, our navigation laws, your fishing bounties, and by making your people our manufacturers, our merchants, our shippers. You are not satisfied with the vast tribute we pay you to build up your great cities, your railroads, your canals. You are not satisfied with the millions of tribute we have been paying you on account of the balance of exchange which you hold against us. You are not satisfied that we of the South are almost reduced to the condition of overseers of northern capitalists. You are not satisfied with all this; but you must wage a relentless crusade against our rights and institutions."
The London Times concurred, "The contest is really for empire on the side of the North and for independence on that of the South...."
So the Confederacy passed laws creating a free trade zone in the South, with no tariffs at all. This was an economic threat to the North of no small magnitude. Without the tariff imposed on all north American ports that accessed the hinterland, trade would flow around the Northern ports through southern ports instead, leaving the North ports vacant, and its industrial centers stagnant, and Northerners had no doubts about it. As stated by the Philadelphia Press on 18 March 1861 demanded a blockade of Southern ports, because, if not, "a series of customs houses will be required on the vast inland border from the Atlantic to West Texas. Worse still, with no protective tariff, European goods will under-price Northern goods in Southern markets. Cotton for Northern mills will be charged an export tax. This will cripple the clothing industries and make British mills prosper. Finally, the great inland waterways, the Mississippi, the Missouri, and the Ohio Rivers, will be subject to Southern tolls."
This free trade zone also reversed Lincoln's willingness to take time and allow the division to heal with the rebellious states. The threat of this free trade zone had to be quelled, and so he sent an armed force to reinforce the Ft Sumter garrison and to collect any tariffs entering Charleston harbor. This is critical to understanding why Lincoln suddenly took a hard tone with the South, because the free trade zone meant the ruin of the North's economy, and it had to be prevented at all costs.
It was after the Morill tariff and the counter by the Confederacy of creating itself as a free trade zone that the secession reached a second crescendo that caused Arkansas, Virginia, North Carolina, and Tennessee to follow in secession.
So to summarize the situation in the words of another neutral observer of the time, Karl Marx, "The war between the North and the South is a tariff war. The war is, further, not for any principle, does not touch the question of slavery, and in fact turns on the Northern lust for sovereignty."