The North did not attack the South; it was the Confederates who initially succeeded from the Union and fired the first shot of the Civil War at Fort Sumter in 1861.
you havent made a good enough hypothetical yet.
there's no lack of slave states around, including ones that the US does business with happily. i think yes, if you made your hypothetical "what if the US had a slaver neighbor" yes, the US would be leaving them alone, other than some economic pressures here and there