The Missouri Compromise formed the plan that:

The Missouri Compromise formed the plan that t he slavery issue became more intense as the U. S. expanded westward and acquired new territories. The question then was whether slavery should be allowed in the new territories. In 1787, the Northwest Ordinance prohibited slavery in the Northwest Territory, the land north of the Ohio River and west of the Mississippi River. In 1820, the Missouri Compromise prohibited slavery above the 36° 30' latitude line in the Louisiana Territory. Many citizens believed that this compromise settled the question of slavery in the territories. The acquisition of new territory through the Mexican War and in Oregon reopened the question for dispute, however. The Northern members of Congress wanted this territory to be free. The Southern members said that settlers in the territory should make the decision for themselves.