History, Social Development: North and South

The regions of the United States that argued about the Mexican War and its aftermath had grown in divergent ways since agreeing to be a nation in 1788. The North had experienced a market revolution based on commercial agriculture and the growth of cities and industry. The South, on the other hand, remained tied to a plantation system that depended on slave labor and international markets. The plantation system enslaved the one-third of all Southerners who were black and excluded more and more poor whites.