The development of the steam ironclad firing explosive shells in the mid-19th century rendered sailing ship tactics obsolete. New tactics were developed for the big-gun Dreadnought battleships. The mine, torpedo, submarine and aircraft posed new threats, each of which had to be countered, leading to tactical developments such as anti-submarine warfare and the use of dazzle camouflage. By the end of the steam age, aircraft carriers had replaced battleships as the principal unit of the fleet. Beginnings in the 19th Century The interval of ninety years between the end of the Napoleonic Wars in 1815 and the beginning of the Russo-Japanese War in 1904 was marked by no major naval war. There was fighting at sea, and there were prolonged blockades, but there were no campaigns between large and well-appointed navies. During this period an entire revolution took place in the means of propulsion, armament and construction of ships. Steam was applied to warships, at first to auxiliary forces, in the second quarter of the 19th century. The Crimean War gave a great stimulus to the development of the guns, and brought about the application of iron to ships as armour-plate. Very soon metal was adopted as the material of which ships were made. The extended use of shells, by immensely increasing the danger of fire, rendered wood so inflammable that it was too dangerous for employment in a warship. Changes so sweeping as these could not take place without affecting all the established ideas as to propulsion, armament and construction. Revival of ramming Steam allowed the ship herself to be used as a projectile. Many thought that the use of the ram would again become common and the sinking of Re d'Italia by the Austrian Erzherzog Ferdinand Max at the battle of Lissa in 1866 seemed to give force to this supposition. Accidental collisions such as those between the British warships Vanguard and Iron Duke, Victoria and Camperdown showed how fatal a wound could be given by the ram of a steam warship. But even the sinking of Re d'Italia was largely an accident, and steam-powered ramming turned out to be impractical. Between vessels both under full control, a collision was easily avoided where there was space to move. In a mêlée, or pell-mell battle, opportunities would occur for the use of the ram, but the torpedo and the mine soon made it very dangerous for one fleet to