If the North had fought the Civil War to free slaves it would have been a just war.
...and if the Confederates hadn’t shelled Fort Sumter, providing Lincoln with the very spark he needed to mobilize for war, how different might things have turned out...
...when examining war, look to see who struck the first actual blow...
That was a dumb move.
But be prepared to explain what qualifies as an "actual" blow. If you're taking aimed fire, then you might safely suspect that there is some reasoning behind it.
In this case, the lead-up was the actions of Major Anderson in spiking the guns of Fort Moultrie and then moving his command to Fort Sumter, where he could cut off the ship channel into Charleston. When Lincoln moved to resupply rather than withdraw him, that maybe wasn't an "actual" blow, but it was definitely a threat.
Lincoln was a lawyer, and he knew how to gain the upper hand in the eyes of a jury.