Imagine an America that diverged from ours at the Constitutional Convention. The 3/5s of a person compromise failed and the United States split into two nations in 1788. The next seventy years could have gone much the same as our own history. The great political compromises were almost treaties between separate regions anyway. An independent North and South would have a Fugitive Slave Treaty, a Treaty of 1820, and a Treaty of 1850. They might even ally against Mexico in a successful war of conquest, with the North taking California as its share of the spoils. So by 1860 the continent might not look that different from our own history, just with two similar Constitutions instead of our one.
Of course, in the North the Abolitionist movement would hold an uncontested majority in the government with no slave-holding states to dilute their influence. There'd be no shortage of sparks to trigger a war--Northerners inciting slave revolts, Southern atrocities in crushing revolts, Bleeding Kansas. So war could break out in 1861 in this scenario, right on schedule. The moral issues would be the same as in our own Civil War, except for the claim of "preserving the Union." So, what's your opinion?
Would the North be justified in launching a war to end slavery in the South?
Yes, slavery is evil and should be abolished
Yes, if victory will be quick and cheap
No, there's no excuse for interfering in another country's internal affairs
No, slavery was just an excuse to get economic control of the South
No, war is too horrible for abolitionism to justify it
No, economic forces will end slavery eventually
Pass, too hard a question