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.
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From http://www.lewrockwell.com/orig/pearlston1.html :
The Republican platform of 1860 called for higher tariffs; that was implemented by the new Congress in the Morill tariff of March 1861, signed by President Buchanan before Lincoln took the oath of office. It 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."
Also see: http://www.lewrockwell.com/rockwell/civilwar.html
The slavery issue was raised by Lincoln later, as a way to create support for the war. In fact, slavery was already on its way out, and would have faded away on its own without the war.
