In the spring of 1861, he served as an aide-de-camp to General P.G.T. Beauregard and was sent by the general to demand the surrender of Fort Sumter in Charleston. After the commander of the fort, Major Robert Anderson of the U.S. Army declined to surrender, Chesnut gave orders to the nearby Fort Johnson to open fire on Fort Sumter. In consequence the first shots of the Civil War were fired, on April 12, 1861. In the summer of 1861 Chesnut also took part in the First Battle of Manassas as an aide-de-camp to Beauregard.