<P> The question of how important the tariff was in causing the war stems from the Nullification Crisis, which was South Carolina's attempt to nullify a tariff and lasted from 1828 to 1832 . The tariff was low after 1846, and the tariff issue faded into the background by 1860 when secession began . States' rights was the justification for nullification and later secession . The most controversial right claimed by Southern states was the alleged right of Southerners to spread slavery into territories owned by the United States . </P> <P> Under Lincoln's leadership, the war was fought to preserve the Union . With slavery so deeply divisive, Union leaders by 1862 reached the decision that slavery had to end in order for the Union to be restored . Union war evolved as the war progressed in response to political and military issues, and historians do not use them to explain the causes of the war . The key new issues were the destruction of slavery and the legal and economic status of the freed slaves . </P> <P> The deep institution of slavery was the major cause of the American Civil War, with the South seceding to form a new country to protect slavery, and the North refusing to allow that . Historians generally agree that other economic conflicts were not a major cause of the war . Economic historian Lee A. Craig reports, "In fact, numerous studies by economic historians the past several decades reveal that economic conflict was not an inherent condition of North - South relations during the antebellum era and did not cause the Civil War ." When numerous groups tried at the last minute in 1860--61 to find a compromise to avert war, they did not turn to economic policies . </P> <P> The South, Midwest, and Northeast had quite different world 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 pointed out that his image of a unified Northeast was incorrect because the region was highly diverse with many different competing economic interests . In 1860--61, most business interests in the Northeast opposed war . After 1950, only a few mainstream historians accepted the Beard interpretation, though it was accepted by libertarian economists . As Historian Kenneth Stamp--who abandoned Beardism after 1950--sums up the scholarly consensus: "Most historians...now see no compelling reason why the divergent economies of the North and South should have led to disunion and civil war; rather, they find stronger practical reasons why the sections, whose economies neatly complemented one another, should have found it advantageous to remain united ." </P>

What was the main issue in the civil war