What was the main reason northern states opposed slavery?

It was an unfair business advantage, is the right answer.

Explanation:

The antislavery movement that began in the North during the 1830's is more popularly known as the Abolitionism. According to the Abolitionists, slavery was unfair and thus it should be eliminated. The main reason why the Northern opposed slavery was that it was an economic threat to the economy of the North.

the monroe doctrine is the best known u.s. policy toward the western hemisphere. buried in a routine annual message delivered to congress by president james monroe in december 1823, the doctrine warns european nations that the united states would not tolerate further colonization or puppet monarchs.

b.lincoln had the victory he wanted for emancipation