I think your answer would be The Lousiana Purchase.
This is how I got my answer: https://www.loc.gov/rr/program/bib/ourdocs/missouri.htm
"In an effort to preserve the balance of power in Congress between slave and free states, the Missouri Compromise was passed in 1820 admitting Missouri as a slave state and Maine as a free state. Furthermore, with the exception of Missouri, this law prohibited slavery in the Louisiana Territory north of the 36° 30´ latitude line. "(Missouri Compromise)
Work Cited:
"Primary Documents in American History.” Planning D-Day (April 2003) - Library of Congress Information Bulletin, Victor, www.loc.gov/rr/program/bib/ourdocs/missouri.html.
