Trench warfare is a form of occupied fighting lines, consisting largely of trenches, in which troops are largely immune to the enemy's small arms fire and are substantially sheltered from artillery. It has become a byword for attrition warfare, for stalemate in conflict, with a slow wearing down of opposing forces.

Trench warfare occurred when a military revolution in firepower was not matched by similar advances in mobility, resulting in a grueling form of warfare in which the defense held the advantage. 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. Attacks, even if successful, often sustained severe casualties as a matter of course.