Because it was a US Army garrison on an island in Charleston harbour.
The Confederates wanted to assert their sovereignty over the whole of South Carolina. The Union refused to recognise the Confederacy, and wanted to assert their rights over the island and its garrison.
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General PGT Beauregard.the commander of fort Sumter was Robert Anderson
Beauregard was a Confederate. In Charleston, he ordered the artillery to fire the first shots of the war, at the island garrison of Fort Sumter.
Lincoln had ordered supplies sent to the fort.
The USA - soon to be known as 'The Union' for the duration of the war - which did not recognise the newly-formed Confederate States of America.The CSA wanted to assert its sovereignty over the island fortress, as it was in Charleston Harbour, and therefore part of South Carolina, a Confederate state. Their action in firing on Sumter is taken as the start of the war.South Carolina declared its secession from the United States on December 20, 1860. At this time, Fort Sumter was a Union fort in the Charleston harbor.
Lincoln decided to send supplies to Charleston in the first week of April. When the federal relief expedition arrived at Fort Sumter on April 12, Confederate forces opened fire, and the undermanned and unprepared federal troops were forced to surrender the next day.