You know that they were authorized to use force if the resupply mission was resisted by Confederate authorities.
Well everyone knew they were going to be resisted. Lincoln knew it. His cabinet knew it. Major Anderson knew it, so let's move past the point of wondering if they were going to be resisted and move straight to the "What were they going to do?"
More importantly, what did the Confederates believe they were going to do? What was the impression that Lincoln wanted the Confederates to believe?