With Charleston as full of fanatics as it was, you could send boats out to the fort to see what happens. Would the boats be fired on, ensuring that the US fired the first shot? Would the garrison surrender to a larger force without firing a shot? Would there be a lot of men milling around at the gates of the fort wondering what to do?
Once you get away from the idea of a "forced" war or a war by trickery, you realize that there were other options available at the time, if one had the wisdom and patience not to initiate force.
It might be useful to consider what the Confederates might have been concerned about when informed that the Sumter expedition was coming. Could they trust Lincoln's word that there would be no attempt to reinforce the fort if the South allowed the Union to resupply it? Would Lincoln use the occasion to inject 200 men and supplies for a year into the fort even if the South had allowed Lincoln to supply the fort with food?
The South Carolina governor had previously been told by Lincoln's agent Lamon that the fort would be evacuated. Seward had told the Confederate Commissioners in Washington much the same thing. Then suddenly here is this fleet of warships coming South.
The Confederate Commissioners said the following as they were leaving Washington [Source: New Orleans Daily Picayune, April 13, 1861]:
Washington, April 11. -- The Southern Commissioners charge the Administration with gross perfidy in attempting to reinforce Fort Sumter under pretext of evacuation.
They say the Montgomery Government earnestly desires peace. They return convinced that war is inevitable, saying the responsibility rests on the Administration.
I can imagine what would happen if I ignored a rattlesnake's warning rattle and came to close to it. It might bite me, but I couldn't really claim that I was just going to pet it while holding a pistol behind my back and it was the rattlesnake who initiated the hostilities.
Now for a quote from the (Republican press) New York Times in their April 12th edition:
Sumpter [sic] on the one side and the Fleet off the North Channel on the other, will effectively cover any relieving expedition, whether of open boats, tugs, or small vessels, from any maritime attack, and confine all resisting operations to the land batteries. Experience has shown -- as in the case of Gen. WILKINSON’S passage down the St. Lawrence during the last war [the Mexican War doesn’t count as a war in the Times view?], with five hundred boats, suffering but a trifling loss, in the face of strong shore batteries – that batteries cannot effectually prevent the passage of an armament. Still less can be done when the batteries themselves will be exposed to such a terrific fire as Major ANDERSON can for some hours at least, pour with his whole force on Moultrie and the battery near Cummings' Point, the only two places from which boats or light draft vessels can be fired upon to any purpose.
But ANDERSON’S fire will not be the only one to which Moultrie may be exposed, as the smaller vessels can take with impunity positions from which shell may be thrown with great effect. No matter how brave or skillful the Southern troops may be, they will be under a fire which will render the entire stoppage of relief to Fort Sumpter [sic] nearly impossible.
A storm prevented Northern ships from crossing over the Charleston bar and dispersed the Northern tugs that were to take in supplies.
Then the Times said the following:
… Why the Southern Commander, be he JEFFERSON DAVIS or Gen. BEAUREGARD, has delayed pouring on Sumpter [sic] his full force, and crushing it beneath an iron hail, if he could; why he has waited until, instead of concentrating his fire in security on one small point, he now has to defend a long straggling line [ten miles of shoreline], from a powerful fleet, it is impossible to tell. The reason may have been political; it may have been that there was not the vaunted readiness; it may have been incompetency; and it is not impossible that when the yawning abyss opened before them with all its horror, they may have lacked the insane courage required for the final leap.
The Times question shows a lack of understanding of Southern intentions. IMO, if the South had wanted war, they would have struck long before the North was ready just as the Times said and not waited until the Northern fleet was already on the way. The South had Commissioners in Washington trying to negotiate peace until the last moment. They were lied to by the Lincoln administration about the evacuation of Sumter and not officially received by Lincoln.