Lincoln implied the use of force/violence in his address not over the ‘spread’ of slavery but amongst other things the collection of those ‘duties and imports’. Tariffs had been an issue with many of the Southern states for decades. South Carolina threatened succession in 1832 not over slavery but tariffs. Tariffs were a powder keg with the South. Slavery was the fuse. As it appears opinions are fairly set on this subject, I will peacefully ‘secede’ from further comment.
Okay, so I get the last word.
Slavery was the great issue of the 1850s. The country was up in arms over the expansion of slavery (and the threat to freedom it implied) and the spread of abolitionism (and the explicit threat to slavery it carried). Nobody was very excited about the tariff. Nobody was screaming about the tariff in 1860.
That doesn't mean most supporters of the union cared about slavery. They were concerned about the fate of the union. And it doesn't mean that all confederates cared only about slavery. Many of them only supported secession because of Lincoln's call for an army to keep the Deep South states (which largely did secede because of slavery) in the union.
But without slavery, there wouldn't have been a war. The tariff question was one that could have been resolved peacefully. By 1860. the slavery question couldn't. It was only after the war, after slavery became a dirty word that the myth that the war was fought over tariffs came into its own.
And Lincoln himself? This whole caricature of him obsessed by the tariff doesn't ring true. Tariffs were one way you knew which country you were in and where the border was, and the continued existence of the country within the existing borders under the existing laws was what mattered to him.