Fort Sumter was considered the first battle of the Civil War. Before this happened, the Southern states seceded (left) the United States and formed the Confederate States of America. So at this time, the Confederates considered South Carolina part of their territory. This meant that it no longer belonged to the United States.
When the US army refused to leave this site, the Confederate army bombarded them with gun fire. By the afternoon, the US army surrendered.
From this point Lincoln had a decision to make. Either a) try to negotiate and bring the Confederacy back into the US or b) go to war, as this was a severe attack on the US army.
In your writing I would discuss how Lincoln needs to show a sign of strength and to not back down from the Confederacy. If he fails to respond in a strong manner, the Confederacy will take advantage of his weakness in order to spread their power and influence.