Please tell me which military forts the South claimed ownership of that were located in the north after they left the union. That the north claimed any sort of ownership of military forts in southern states that seceded was the initial aggressive act. All the south did was leave the union. That's not being aggressive and they had no plans to attack any union state as they knew a war with the north would be a losing proposition. The north knew this an thus provoked a war.
With all respect, you are mistaken. The South wanted a fight. The forts that the Union rightfully claimed (they were Federal property, and legally remained so even if one granted the legality of secession) were mostly not garrisoned prior to the war. Ft Moultrie had a garrison of 85 men led by Major Robert Anderson. Anderson saw that Moultrie was an indefensible position and moved his garrison to Sumter. He was essentially surrounded by hostile forces and his occupation of the fort was more like a siege by the Confederates. The fort was cut off from supplies and rations and ammunition were in short supply. An attempt was made to supply the garrison via a merchant ship, but the firing of shore batteries drove the supply ship away.
The point is that the South has no military reason to fire on Sumter or demand its surrender. A garrison of 85 men in Charleston harbor posed no real threat. Had Anderson attempted to use his position to fire on ships bound for Charleston, he would have been wiped out by the surrounding Confederate forces. The possession of the fort was nothing more than a point of honor; the South refused to allow it and the North refused to abandon it. Either side could have averted the war (at least temporarily) by backing down. Both sides refused to do so.
You are also mistaken if you think that the South believed they would lose. That’s hindsight talking. At war’s start the South was quite confident that they had better soldiers and better leaders than the North. Events mostly proved that assessment correct, at least for the first two years of the war. The losses at Gettysburg and Vicksburg, with hindsight, marked the turning point, but even through 1863, Southern confidence in ultimate victory was quite high. It was not until the capture and burning of Atlanta, Sherman’s march to the sea, and the seige warfare at Petersburg that Southern confidence waned. Certainly in April 1861 they thought they would win.