Even the romans was using Chemical warfare.
In a narrow tunnel under the ruins of a stronghold in the city of Dura, today in Syria. researchers from the Brittish University of Leicester have found the remains as proff of the first chemical warfare in history.
Its about remains of twenty roman soldiers, that shows traces of beeing killed by some kind of gas.
In the middle of 200-century a Persian army where occupying the city, which in that time where a Roman stronghold. The Persians dugg tunnels under the city wall to make them fall, the roman soldiers answerd whit own tunnels in efforts to repell the enemy. It was in one of thoose tunnels that the Persians used the Chemical weapon. When the roman soldiers enterd the tunnel the Persians light a fire to start filling the tunnel whit gas, the Roman soldiers where killed instantly. The Persians was later able to invade the city and killed or deported all the inhabitans of the city.