and nursed Union and Confederate troops at field hospitals. Mary Edwards Walker, the only woman ever to receive the Medal of Honor, served in the Union Army and was given the medal for her efforts to treat the wounded during the war. Her name was deleted from the Army Medal of Honor Roll in 1917 (along with over 900 other Medal of Honor recipients); however, it was restored in 1977. Naval tactics The small U.S. Navy of 1861 was rapidly enlarged to 6,000 officers and 45,000 sailors in 1865, with 671 vessels, having a tonnage of 510,396. Its mission was to blockade Confederate ports, take control of the river system, defend against Confederate raiders on the high seas, and be ready for a possible war with the British Royal Navy. Meanwhile, the main riverine war was fought in the West, where a series of major rivers gave access to the Confederate heartland. The U.S. Navy eventually gained control of the Red, Tennessee, Cumberland, Mississippi, and Ohio rivers. In the East, the Navy shelled Confederate forts and provided support for coastal army operations. The Civil War occurred during the early stages of the industrial revolution. Many naval innovations emerged during this time, most notably the advent of the ironclad warship. It began when the Confederacy, knowing they had to meet or match the Union's naval superiority, responded to the Union blockade by building or converting more than 130 vessels, including twenty-six ironclads and floating batteries. Only half of these saw active service. Many were equipped with ram bows, creating "ram fever" among Union squadrons wherever they threatened. But in the face of overwhelming Union superiority and the Union's ironclad warships, they were unsuccessful. In addition to ocean-going warships coming up the Mississippi, the Union Navy used timberclads, tinclads, and armored gunboats. Shipyards at Cairo, Illinois, and St. Louis built new boats or modified steamboats for action. The Confederacy experimented with the submarine , which did not work satisfactorily, and with building an ironclad ship, , which was based on rebuilding a sunken Union ship, . On its first foray, on March 8, 1862, Virginia inflicted significant damage to the Union's wooden fleet, but the next day the first Union ironclad, , arrived to challenge it in the Chesapeake Bay. The resulting three-hour Battle of Hampton Roads was a draw, but it proved that ironclads were effective warships. Not long after the