 The compromise was that Maine would enter the Union as a free state to balance Missouri.Of the greatest concern for Missouri slave-holders in the years before the war was a federal law that decreed that if a slave physically entered a free state, the slave became free.The Underground Railroad, a network of safe houses through which runaway slaves could find protection and refuge while heading north, was already established in the state, and slave owners were worried about the possibility of Missouri's entire western border becoming a conduit for the Underground Railroad if adjacent territories were made free states.