Motor vehicle with plating for protection against bullets, shells, or other projectiles that moves on wheels or tracks. The tank is the chief armoured vehicle for larger military forces. Other military types include infantry fighting vehicles, amphibious landing vehicles, and mobile weapons platforms such as self-propelled artillery and antiaircraft guns. Infantry fighting vehicles, descended from the armoured personnel carriers of World War II and the Vietnam War, are armoured, tracked vehicles that transport infantry into battle but also serve as platforms from which soldiers can fight without dismounting. Armoured cars are wheeled civilian vehicles, ranging from commercial trucks to luxury sedans, that generally are equipped with armour and other amenities for securely transporting valuables and individuals over paved roads.
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The roots of the modern AVLB can be found in World War I, at the dawn of tank warfare. Having developed tanks, British and French were confronted with the problem of mounting tank advances in the face of the trenches that dominated the WWI battlefields. Early engagements, such as at Cambrai demonstrated the tank's utility, but also highlighted its vulnerability to battlefield geography -- many early tanks found themselves ignominiously stuck in the trenches, having insufficiently long tracks to cross them (as at right). To counter this disadvantage, tanks (especially the common British Mark series) began to go into battle with fascines hanging over their bows, sometimes as simple as a bundle of heavy sticks. By dropping these into the trenches, they were able to create a wedge over which the tank could drive. Later, some tanks began to carry rails on their decks -- the first AVLBs.
One of the earliest series-produced examples is the Brückenleger IV, a German AVLB, which entered Wehrmacht service in 1940; 20 were built. Problems of excessive weight limited the vehicle's effectiveness, and eventually all 20 were converted back to tanks, but the trend was underway. The Allies developed similar equipment, mostly based on the ubiquitous Churchill and Sherman medium tanks of the British and U.S. armies, respectively. In some early designs, bridge-layers could emplace bridges, but not retract them. Other AVLBs were integral to the bridge themselves, wading to the middle of a river and extending simple ramps in both directions (such as the Churchill Ark); following vehicles would drive directly over the bridge-layer.
Having proven their utility in the war, and being especially useful for amphibious operations, many major tank operators (at least, those with rivers or similar obstacles to cross) employ some number of bridgelayers, usually converted from existing tank designs (sometimes using old tank designs no longer useful for front-line combat service, as in the M60A1 AVLB. A number of designs, including the French PTA 2, now use 6x6- or 8x8-wheel trucks as their base instead of tracked chassis.