Why was fort Sumter an important battle?

As the Confederacy was organizing itself in the early months of 1861, they were able to take control of Federal forts without bloodshed. And, President Lincoln had not yet taken office. Fort Sumter and the battle to capture it began a quick process that sparked the US Civil War. Sitting in the Charleston, South Carolina harbor, the fort commander refused to surrender. When finally he had no choice, President Lincoln called for 75,000 volunteers to enlist for 3 months and end the Southern rebellion. With this action four other Southern states, most importantly Virginia decided to join the Confederacy. It was now eleven states strong.