Thomas Jefferson, the author of the Land Ordinance of 1784
There were four main parts of the Land Ordinance of 1784: all new states west of the Appalachian mountains will always be part of the United States; they will have the same powers and positions as the original 13 states; they will pay off the national debt equal to the original states, and they would have republican governments.
However, at the beginning there had been a fifth point, stating that after the year 1800, no state west of the Appalachians would have slavery. This final part of the Ordinance was defeated by a single vote, due to the fact that one of New Jersey's representatives was sick that day.
What if New Jersey had been able to cast their vote, and sign this ordinance into law? Every new state would have been slave-free, resulting in the slave states being a much small minority. They would not have fought a civil war over this detail, as the slave-owners already within slave states would have been allowed to keep their slaves.