First off, if this challenge has been done often and/or recently, I am very sorry.
Having said that, the challenge is simple: come up with a scenario that would result in the Northern Free States seceding from the Union over slavery.
A few rules:
1. New England and only New England doesn't count. It has to include other states beyond those six.
2. I won't declare an official earliest POD beyond 1789 (when the Constitution went into effect), but I will give a cookie to whomever comes up with one during or after the Missouri Compromise.
3. The Union doesn't necessarily have to go to war, but secession is a must. Also, the North can be separate as long as you want it (a few years, a few decades, until the modern day).
4. The secession can be at any date after the Constitution being made, although a chocolate cookie to whomever has it happen in the 1860s.
5. A triple-chocolate double-stuffed cookie with fresh milk from the animal of your choice (including favorite female celebrities) to whomever has President John Wilkes Booth assassinated by actor Abraham Lincoln!