I assume you mean primarily World War One. Trench warfare developed over time, because at some point, neither side was 'winning' or 'losing'. A stalemate developed, and each side literally 'dug in' to hold the ground they had occupied. The trenches also protected a soldier against the massive amount of machine gun fire, and artillery fire. The trenches became ever larger and more elaborate. Living conditions for the average soldier were terrible. World War Two saw fewer examples of trench warfare, because of greater mobility, and better communication.
World War 1 introduced aerial warfare with airplanes and trench warfare
"Trench warfare was used in World War I"
Trench Warfare
Trench Warfare
trench warfare chemical warfare
Trench warfare. They would sit in trenches and shoot at the enemy trench
Trench warfare. They would sit in trenches and shoot at the enemy trench
Trench Warfare
trench warfare
Trench warfare.
World war 1
A war of movement.