The threat was to the expansion of slavery into the territories, and Lincoln's speeches and the 1860 Republican platform make their opposition to that abundantly clear. None of the Republican leaders were under any illusions that they could end slavery in states where it existed any time soon. The need for a constitutional amendment to accomplish that made it impossible. But they did believe that the flawed Dred Scott decision could be overturned and would certainly had worked towards doing that.
The south chose independence and that’s when the hostilities began.
Hostilities began when the confederacy chose to bombard Sumter. Prior to that there had been no hostile acts on the part of the federal government.
If you believe “Honest Abe Lincoln”, he stated as much 16 months after the initiation of the conflict.
Stated what?
Please refer to his letter to Horace Greeley as cited earlier.