The South, Midwest, and Northeast had quite different economic structures. They traded with each other and each became more prosperous by staying in the Union, a point many businessmen made in 1860–61. However, Charles A. Beard in the 1920s made a highly influential argument to the effect that these differences caused the war (rather than slavery or constitutional debates). He saw the industrial Northeast forming a coalition with the agrarian Midwest against the plantation South. Critics challenged his image of a unified Northeast and said that the region was in fact highly diverse with many different competing economic interests. In 1860–61, most business interests in the Northeast opposed war.