Chemical Agents
Chemical agents are chemical mixtures used to disrupt enemy combatants in order for your troops to move in. The most common was mustard gas which was a mixture of chlorine and iodine gasses that caused rashes, blisters, and blindness in its victims and it wasn't choosy with those either. Often once it was deployed (usually by artillery) it would cause massive to its targets but if the wind picked up often the opposition was handed its own medicine.