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temporary bridge that must usually be constructed in haste by military engineers, from available materials, frequently under fire. The earliest types historically were pontoon bridges-i.e., floating bridges that rest on stationary boats. Pontoon bridges were constructed in ancient times by Persians, Greeks, Romans, and Mongols, the most famous being Xerxes' 2-mile (3-kilometre) span over the Hellespont (Dardanelles). The Romans, however, frequently built more durable military bridges, notably Julius Caesar's timber crossing of the Rhine River and Trajan's timber-arch span of the Danube. By the 17th century, bridging equipment was part of the train of European and Turkish armies, with pontoons fashioned not only of timber but also of leather, copper, and tin. Pneumatic pontoons with steel roadways made their appearance during World War II.