American Civil War: The H. L. Hunley becomes the first submarine to engage and sink a warship, the USS Housatonic.

The American Civil War was a civil war in the United States between the Union and the Confederacy, which was formed in 1861 by states that had seceded from the Union. The central conflict leading to war was a dispute over whether slavery should be permitted to expand into the western territories, leading to more slave states, or be prohibited from doing so, which many believed would place slavery on a course of ultimate extinction.

H. L. Hunley, also known as the Hunley, CSS H. L. Hunley, or CSS Hunley, was a submarine of the Confederate States of America that played a small part in the American Civil War. Hunley demonstrated the advantages and dangers of undersea warfare. She was the first combat submarine to sink a warship (USS Housatonic), although Hunley was not completely submerged and, following her attack, was lost along with her crew before she could return to base. Twenty-one crewmen died in the three sinkings of Hunley during her short career. She was named for her inventor, Horace Lawson Hunley, shortly after she was taken into government service under the control of the Confederate States Army at Charleston, South Carolina.

The Sinking of USS Housatonic on 17 February 1864 during the American Civil War was an important turning point in naval warfare. The Confederate States Navy submarine, H.L. Hunley made her first and only attack on a Union Navy warship when she staged a clandestine night attack on USS Housatonic in Charleston harbor. H.L. Hunley approached just under the surface, avoiding detection until the last moments, then embedded and remotely detonated a spar torpedo that rapidly sank the 1,240 long tons (1,260 t) sloop-of-war with the loss of five Union sailors. H.L. Hunley became renowned as the first submarine to successfully sink an enemy vessel in combat, and was the direct progenitor of what would eventually become international submarine warfare, although the victory was Pyrrhic and short-lived, since the submarine did not survive the attack and was lost with all eight Confederate crewmen.