Halabja poison gas attack happened on 1988-03-16.

I am not 100% sure but this is very similar to the Sarin Gas attack in the Tokyo subway.

Poison gas was first used during this battle

A poison gas attack using gas cylinders in world war one

Poison gas was never a decisive military weapon, since it is very unpredictable. A change in the direction of the wind can cause the gas that you were using to attack the enemy, to blow back in your direction and kill you instead. Nonetheless, poison gas is still feared, and armies continue to be equipped with gas masks, just in case.