Okay, here’s one closer to home you might know about. Cuba wants Guantanamo back. They claim that the treaty signed with the previous government is null and void, they refuse to cash annual rent checks and have repeatedly demanded the US leave what they consider Cuban soil. If they begin shelling the fort, are they justified? Is it an act of war?
It is human nature to force everything we deal with into neat little Cubbyholes because yes/no decisions are easy to understand. Percentages and probabilities are not. Much of mathematics is efforts to simplify complex equations into simple, yes/no answers.
We want definite boundaries of transition between one state and another, we want clear lines of demarcation, we want go or no go solutions to our equations. We want binary answers, though the world is full of analogue problems.
If Cuba fires an rpg at Guantanamo, is that an act of war? If they fire a dozen, is that an act of war? How many mortars have landed across the DMZ in Korea, and it's still not a war?
I think a reasonable answer as to whether or not something is an act of war will depend quite a lot on circumstances beyond just one aspect.
I think the boundary designating an "act of war" will move, depending on who is firing what at whom for how long and for what reasons. Cuba and the Southern states are not analogous. You'll grant your Brother indulgences that you wouldn't grant a stranger.
Had it been in Lincoln's mind to ignore the attack, he could have done so. If the President told everyone to stand down, that's what would have happened. It wasn't and he didn't.
No, the Confederates made the stupidest possible blunder by taking that toy away. They should have let the large kid play with it till he got tired of it, and left.
Their pride simply would not be restrained, and many people thereafter suffered for it.