Lincoln was landing men at Ft. Sumter. He was going to reinforce the fort. Had they done nothing, they would have been facing even more soldiers and guns at Sumter than they were before.
The act of attempting to land those men to reinforce Sumter, after assuring everyone that he would not, was an act of War.
Even Major Anderson more or less said it was a chickensh*t underhanded move.
But this argument is pointless. If you think that just having a federal garrison in a federal fort was an unendurable assault on South Carolina that justified starting a war then you're going to start a war -- or justify starting the war. If you believe you're justified in firing first and fire first, you've already started the war -- and lost the argument.
PS, What happened to all those New England cotton men who you say were so passionate for war? Haven't heard much about them lately.
Rubbish.
First of all, Lincoln assured nobody he would abandon Fort Sumter.
Yes, he did offer in negotiations with Virginia Unionists to swap "a fort for a state" -- if Virginia agreed to adjourn its secession convention and remain in the Union, then Lincoln would withdraw from Fort Sumter.
But Virginians refused Lincoln's offer, and so it was withdrawn.
Second, Lincoln announced his resupply mission to Fort Sumter directly to South Carolina Governor Pickens.
He promised Pickens (and ordered his own commander) there would be no reinforcement if the resupply mission was met with no force from Confederates.
Third, the analogy with Guantanamo Bay applies here: regardless of what the Communist Cuban government may say about Gitmo, no US mission to resupply or reinforce US troops on that US base can be considered an act of war.
But any Communist or Confederate assault of US forces is an act of war, period.