The debate was whether or not the United States should keep slavery. It led to the civil war because the northerners wanted to abolish slavery while the southerners wanted to keep it, threatening to secede from the Union if their right as a state to have slaves was abolished. Who had the most to win is debatable, but you could say that the north did. Not only would they preserve the union and free the slaves, but it would also enable them to have access to the crops and agriculture and economy of the south since they would take over, but they would also have an immense support of new voters in the form of African Americans who got voting rights with the reconstruction amendments.