Trench warfare is a form of occupied fighting lines, consisting largely of trench

Trench

A trench is a type of excavation or depression in the ground. Trenches are generally defined by being deeper than they are wide , and by being narrow compared to their length ....

es, in which troops are largely immune to the enemy's small arms

Small arms

Small arms is a term of art used by armed forces to denote infantry weapons an individual soldier may carry. The description is usually limited to revolvers, pistols, submachine guns, carbines, assault rifles, battle rifles, multiple barrel firearms, sniper rifles, squad automatic weapons, light...

 fire and are substantially sheltered from artillery

Artillery

Originally applied to any group of infantry primarily armed with projectile weapons, artillery has over time become limited in meaning to refer only to those engines of war that operate by projection of munitions far beyond the range of effect of personal weapons...

. It has become a byword for attrition warfare

Attrition warfare

Attrition warfare is a military strategy in which a belligerent side attempts to win a war by wearing down its enemy to the point of collapse through continuous losses in personnel and matériel....

, for stalemate in conflict, with a slow wearing down of opposing forces.

Trench warfare occurred when a military revolution

Revolution in Military Affairs

The military concept of Revolution in Military Affairs is a theory about the future of warfare, often connected to technological and organizational recommendations for change in the United States military and others....

 in firepower

Fire power

Firepower is the military capability to direct force at an enemy. It is not to be confused with the concept of rate of fire, which describes cycling of the firing mechanism in a weapon system. It involves the whole range of potential weapons...

 was not matched by similar advances in mobility

Mobility (military)

Mobility in military terms refers to the ability of a weapon system, combat unit or armed force to move toward a military objective. Combat forces with a higher mobility are able to move more quickly, and/or across more hostile terrain, than forces with lower mobility.Mobility is regarded as a...

, resulting in a grueling form of warfare in which the defense held the advantage.