vessel which carried sails to supplement its steam engines into the steel-built, turreted battleships and cruisers familiar in the 20th century. This change was pushed forward by the development of heavier naval guns (the ironclads of the 1880s carried some of the heaviest guns ever mounted at sea), more sophisticated steam engines, and advances in metallurgy which made steel shipbuilding possible. The rapid pace of change in the ironclad period meant that many ships were obsolete as soon as they were complete, and that naval tactics were in a state of flux. Many ironclads were built to make use of the ram or the torpedo, which a number of naval designers considered the crucial weapons of naval combat. There is no clear end to the ironclad period, but towards the end of the 1890s the term ironclad dropped out of use. New ships were increasingly constructed to a standard pattern and designated battleships or armoured cruisers. Trains Armoured trains saw use during the 19th century and early 20th century in the American Civil War (1861–1865), the Franco-Prussian War (1870–1871), the First and Second Boer Wars (1880–81 and 1899–1902), the Polish–Soviet War (1919–1921), the First (1914–1918) and Second World Wars (1939–1945) and the First Indochina War (1946–1954). The most intensive use of armoured trains was during the Russian Civil War (1918–1920). Armoured fighting vehicles Ancient siege engines were usually protected by wooden armour, often covered with wet hides or thin metal to prevent being easily burned. Medieval war wagons were horse-drawn wagons that were similarly armoured. These contained guns or crossbowmen that could fire through gun-slits. The first modern armoured fighting vehicles were armoured cars, developed circa 1900. These started as ordinary wheeled motor-cars protected by iron shields, typically mounting a machine gun. During the First World War, the stalemate of trench warfare during on the Western Front spurred the development of the tank. It was envisioned as an armoured machine that could advance under fire from enemy rifles and machine guns, and respond with its own heavy guns. It used caterpillar tracks to cross ground broken up by shellfire and trenches. Aircraft With the development of effective anti-aircraft artillery in the period before the Second World War, military pilots, once the "knights of the air" during the First World War, became far more vulnerable to ground fire. As a response, armour plating was added to aircraft to protect aircrew and