One of the common myths that American kids are taught in school is that the Civil War was fought over slavery. That's incorrect: the war was not started because of the North's objection to the institution of slavery. During his inaugural address, Lincoln said he would not interfere with slavery wherever it existed, and the south abolished African slave trade as part of the Confederate Constitution. Slavery was also legal in the north during the war.
The war was started because the north tried to impose heavy tariffs on the south. When the south refused to pay, the north responded by threatening to blockade southern ports—which is an act of war.
The Republican platform of 1860 called for higher tariffs. Those tariffs were implemented by the new Congress in the Morill tariff of March 1861, signed by President Buchanan before Lincoln took the oath of office. Morill imposed the highest tariffs in US history, with over a 50% duty on iron products and 25% on clothing; rates averaged 47%. The nascent Confederacy followed with a low tariff, essentially creating a free-trade zone in the South.
Prior to this "war of the tariffs," most Northern newspapers had called for peace through conciliation, but many now cried for war. 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."
The slavery issue was raised by Lincoln later, as a way to create support for the war and as a way to recruit more soldiers. In fact, slavery was already on its way out, and would have faded away on its own without the war.