The development of explosive artillery shells made the use of iron armor plate on warships necessary. In 1859 France launched "Gloire", the first ocean-going ironclad warship. She had the profile of a ship of the line, cut to one deck due to weight considerations. Although made of wood and reliant on sail for most journeys, "Gloire" was fitted with a propeller, and her wooden hull was protected by a layer of thick iron armor. "Gloire" prompted further innovation from the Royal Navy, anxious to prevent France from gaining a technological lead. Meanwhile, smaller ironclad warships became the main tool of the United States Navy in river combat in the American Civil War. In 1906 the new HMS Dreadnought made all older battleships obsolete.