William M. Fowler, "Dahlgren, John Adolphus Bernard," American National Biography Online, February 2000, http://www.anb.org/articles/04/04-00291.html.
As commander of the Blockading Squadron, Dahlgren's chief goal was the capture of the port of Charleston. His predecessor, S. Francis Du Pont, had tried and failed in this attempt and was recalled as a result. During Dahlgren's command Charleston was kept under siege for almost two years. Pressed at sea by Dahlgren's forces and from the land side by William Tecumseh Sherman's advancing army, the Confederates evacuated Charleston 17-18 February 1865. Dahlgren was ordered to transfer his command of the squadron 9 June, when the force was integrated with the Northern Blockading Squadron and returned to its prewar status as the Atlantic Squadron.