North and south civil war?

In the first 19th century,southerners complained of the poor soil and the exhausted lands they used for their plantations.Consequently,they opted for new territories to expand their business.So far,even before invading new mexico and other lands that were under the ownership of mexico,it has been decided that they would be slave states holding agrerian economy.However,Notherners accepted to undergo this war ispite of their convictional objecion to slavery and they signaled only in the aftermath of the war that these new territories are to be free. My question is the following;Why did northerners accept to warry next to southerners in a battle triggered obscenely for uprooting system in the South?