Now there's a little story to that. Chief Justice Taney, in the Dred Scott decision—which said that the Missouri Compromise restriction of slavery in 1820 and any other one, was unconstitutional—said that there was no power in the Congress to forbid slavery in the territories. And he added as a kind of obiter dictum that the only power of Congress over slavery in the territories was the power coupled with the duty of protecting the owner and his rights. Now the seven states of the Deep South interpreted that to mean that the police power of the federal government had to guarantee the integrity of the property of any slave owner going into any United States territory.

The Real Abraham Lincoln: A Debate (2002)