If you look back in this very thread a few pages, I provided evidence that Lincoln and his cabinet deliberately provoked South Carolina into firing on the fort so they could have their casus belli. Long story short, Major Anderson occupied the fort AFTER the declaration of succession. South Carolina had asked them to leave, and had sent delegates to Washington to pay for the property. Lincoln, against the advice of his entire cabinet except his postmaster, of all people, approved a plan (developed by the postmaster’s son-in-law, a naval officer) to send a ship to “Reinforce” Sumter as a deliberate act of provocation. Lincoln also requested that the ship carry nothing but food, so that he could claim that South Carolina attacked an unarmed ship and have the moral high ground.
Some more background info, Major Anderson was purchasing provisions from the town of Charleston well after the declaration of succession, and it was not until Lincoln, in his inaugural address, said that the Southern states could either rejoin the union by choice, or rejoin the union by force of arms, and Anderson moved his force to Sumter, that they refused to sell him food anymore.
In playground terms, Lincoln walked up to South Carolina and called his mother a hooker and told him he was gonna whoop his butt if he didn’t come back to basketball court, then went and ran to the teacher saying “he hit me!” when South Carolina punched him in the face.