Their continued passage of laws concerning the return of escaped slaves, which were contrary to both what they agreed to when they signed the Constitution as well as the legal precedent set by the USSC Court of Appeals.
The Fugitive Slave Act ended all of that and that was passed by congress 6 years before the Civil War. The US military could and did drag those poor people looking for freedom back to the Dixie Hell Hole you so admire.
What's your next lame excuse?