In 1820 an agreement called the Missouri Compromise was reached. The compromise allowed Missouri to come into the Union as a slave state and Maine would be a free state. Congress drew an imaginary line across the middle of the United States running from the east coast to the Pacific Ocean. This imaginary line separated the states into free and slave states. Any new state entering the Union that was south of the line would be a slave state. Any state north of the line would enter the Union as a free state.