Background
In World War I, both sides constructed elaborate trench and dugout systems opposing each other along a front, protected from assault by barbed wire. The area between opposing trench lines (known as "no man's land") was fully exposed to artillery fire from both sides.
What is it?
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Trench warfare is a form of using occupied fighting lines consisting largely of trenches, in which troops are significantly protected from the enemy's small arms fire and are substantially sheltered from artillery.