Whether slavery is viewed as a legal, political, or social problem, then describe the problem the Northern states had with it after secession.
Before the 1857 Dred-Scott decision, Northern states knew with certainty that they could outlaw slavery within their own borders.
They also saw that slavery was seriously declining in Border States like Delaware, Maryland and Missouri.
So there was realistic hope that slavery itself could be gradually abolished.
Based on that, about half of Northerners were content to vote in alliance with Southern Democrats -- to preserve the Union, and the domination of the Slave Power over Federal Government in Washington, DC.
But Dred-Scott turned everything on its head -- not only was slavery not to be abolished by Border States, but now it could not be forbidden in Western Territories or even in Northern States themselves.
That act helped abolish the old Whig Party, and turned many Northern Democrats into abolitionist Republicans.
In sum: by 1860 Northerners were content, as a condition of Union, to allow slavery in the South, but would not tolerate it being imposed on their own states or western territories.
After secession:
At first, many Northerners (especially Democrats) were willing to make almost any concession necessary to preserve the Union, including making slavery lawful everywhere.
But no Congressional proposal was ever even considered, much less accepted, by the new Confederacy, which quickly went the other way: first provoking, then starting (Ft. Sumter), then declaring war on the United States, May 6, 1861.
So there was to be no political compromise to save the Union.
Once the Confederacy declared war on the United States, now the wheel turned yet again.
Almost immediately, Union armies began freeing runaway slaves under the rubric of "enemy contraband".
When they looked to President Lincoln for guidance on this, he considered it in stages, eventually releasing his Emancipation Proclamation in 1862.
That was a happy marriage of military necessity and abolitionist Republican ideological beliefs.
Soon after Lincoln's Emancipation Proclamation, Republicans in Congress began work on a constitutional amendment to abolish slavery in all states, forever.
So what began as an existential war to prevent destruction of the United States ended as war to complete our Founders' unfinished work regarding the Declaration's pledge that, "all men are created equal".
What do you disagree with here?