Poison Gases
Ww1 was the first war that utilized chemical warfare. The first gas attacks used tear gas, which had been invented two years before the war began for the police force. However, due to it's lack of sustained effects and lethality, it wasn't the most effective of weapons against a group of advancing soldiers. The attacks made by the French in August of 1914 did have some effect though - they paved a way for a whole new type of warfare.