What factor most contributed to the huge casualties suffered by the combatants in world war 1?

The massive casualties in World War I were primarily due to the combination of outdated military tactics and advanced industrialized weaponry. The use of trench warfare, combined with machine guns, artillery, and chemical weapons, created a deadly stalemate that resulted in high loss of life. Additionally, the scale of mobilization and the involvement of multiple nations amplified the scale of the conflict and its human costs.