You have command of the Forces surrounding Ft. Sumter. You have been informed that a large group of battleships has been sent from New York with orders to attack you if you do not cooperate with efforts to resupply, perhaps reinforce the fortress.
You have contacted the commander of Ft. Sumter, and offered to maintain a truce with him so long as he does not fire upon your forces. He informs you that he *WILL* fire on your forces if they engage the fleet sent to attack you.
You will be faced with cannon fire from both the fort and from the Warships off the coast. Do you do nothing and wait till both forces attack you simultaneously, or do you try to neutralize one before the other gets here?
Firing on Ft. Sumter was not a mistake. It was the only rational thing someone could do in Beauregard's position.
The war was triggered when Lincoln sent the warships to force the South to submit to the continued presence of troops under Washington DC's control commanding the entrance to one of their most important harbors.
Major Anderson said this would trigger a war, and all of of Lincoln's cabinet but one told him this would trigger a war. Lincoln knew very well he was going to trigger a war, and he did it anyway.
The south lost. Hence, firing on Fort Sumter was a mistake. So was Hitler invading Russia and Japan bombing Pearl Harbor. Here’s a rule: starting a war that you lose is a bad idea. It’s called a miscalculation.
You forgot to add that Davis’s cabinet told him firing on Fort Sumter would cause a war. Davis disregarded this and ordered Beauregard to reduce the fort, by force if necessary, before the supply ships and their escorts arrived of the entrance to Charleston.