I will try to give a better, short answer, but this topic is complex and can not be summarized in 4 sentences. Before 1865 the labor market saw goods manufactured by freeborn labor working for wages in the North making industrial and other goods (metal items, furniture, etc). The end price includes $ for paying the worker. In the South each mega plantation made its own private furniture and farm equipment. The slave owner did not pay his slave to work: slave labor is unpaid. Brissa, you own a chair making company in PA and want to sell your chairs to many customers in the next 10 states. Why won't Southerners buy your chairs? Because they make their slaves make chairs for free and won't pay you unless you sell something they can't make like a ship. You get mad that you cannot make any sales when competing with slave labor. This is why Northern Industrialists and workers opposed slavery - it was not really about immorality. Since Northerners had more people and more seats in Congress but were unable to succeed due to Southern opposition. And the South had extra "slave seats" in the HoR due to the 3/5ths Compromise, which made it harder to pass any laws. Then they overturned the Gag Rule, since after 1850, even counting the "slave seats," the North had double the seats and began to pass bills regulating slavery, Southern senators protected slavery, but when CA and OR and MN entered the Union as Free states, everyone could see the Free states would eventually break the pro-slavery balance in the Senate (a few Northern pro-slavery senators had to be pushed out). Reality: about 25% of the people had 33% of the HoR seats and 30/66 in Senate. The South was holding down the other 75% to preserve slavery and unfair labor practices. Fremont in 1856 and Lincoln in 1860 ran for President, opposing this. Both suggested adding new states in the West which would bring more Free state seats to Congress to overwhelm the pro-slavery minority and thereby change the law. The South had run out of territory into which they could expand (one idea was to take Cuba), so they could not easily get more seats in Congress, but the North could (CA, OR, MN, KS, Neb., Dakotas). Lincoln won the 1860 election with almost no votes from the South but almost the entire North (only half of NJ). In some southern states Lincoln was left off the ballot. But he won anyway. Before his inauguration war threatened. Many statesmen tried to set up compromises to save the Union, but the southern leaders preferred to leave a country than accept Lincoln as President, because they resented his desire to contain slavery to its current status. They saw the repudiation of the Compromise of 1850 in the Dred Scott decision as a right to have slavery in PA, Mass, MN, NY, and everywhere, arguing that Chief Justice Tawney had ruled Congress lacked the power to limit or prevent slavery anywhere, whereas Lincoln insisted the Executive and Legislative branches together had that right. The South tacitly gave this ultimatum to the North: you allow slavery everywhere or we are leaving the Union and making our own pro-slavery nation. Lincoln said no, and you can't leave. They left and we had war.