Warfare in which the opposing sides attack, counterattack, and defend from sets of trenches dug into the ground. It was developed by Sébastien Le Prestre de Vauban in the 17th century for laying siege to fortresses. Its defensive use was first institutionalized as a tactic during the American Civil War. It reached its highest development in World War I. Little used in World War II, it reappeared in the Iran-Iraq War in the 1980s. A typical construction consisted of two to four parallel trenches, each dug in a zigzag, protected by sandbags, and floored with wooden planks. The parallel trenches were connected by a series of communication trenches dug roughly perpendicular to them. The first row was fronted by barbed wire and contained machine-gun emplacements; the rear trenches housed most of the troops. Increased use of tanks marked the end of trench warfare, since tanks were invulnerable to the machine-gun and rifle fire used by entrenched soldiers.
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Any long, narrow, steep-sided depression in the ocean bottom in which maximum oceanic depths (24,000–36,000 ft, or 7,000–11,000 m) occur. The deepest known depression of this kind is the Mariana Trench. Most trenches occur at subduction zones, where one tectonic plate is thrust under another.
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A trench is a type of excavation or depression in the ground. Trenches are generally defined by being deeper than they are wide (as opposed to a wider gully or ditch), and by being narrow compared to their length (as opposed to a simple hole).
While trenches have often been dug as defensive measures, in the pre-firearm eras, they were mainly a type of hindrance for an attacker of a fortified location, such as the moat around a castle (this is technically called a ditch).
Only with the advent of accurate firearms, and the tactics that evolved in World War I and the Crimean War, did the use of trenches as positions for the defender of a fortification become common, though the Māori of New Zealand were known to have used it earlier in their Pā fortifications in the late 19th Century. The military usage evolved very quickly in the First World War, until whole systems of extensive main trenches, backup trenches (in case the first lines were overrun) and communication trenches had been developed, often stretching dozens of kilometres along a front without interruption, and some kilometres further back from the opponents' lines.