Complex sentence: The mechanized mass-destruction of enemy combatants grew ever more deadly: in World War I (1914-1918) machine-guns, barbed wire, chemical weapons and land-mines entered the battlefield.
Split into simpler sentences: The mechanized mass-destruction of enemy combatants grew ever more deadly. In WWI (1914-1918) machine-guns, barbed wire, chemical weapons and land-mines entered the battlefield.