iraq used mustard gas (slow dissipating) to shell the countryside of halabja to impede iranian movements. iran used cyanide (fast dissipating) inside halabja to wreck the iraqi garrison there and allow their storm units to quickly seize the town.
adding to that, anfal was a series of military operations by kurdish tribal forces aligned with the govt organized as light infantry battalions specifically targeting rebelling paramilitary organizations like the puk. the goal of the operation was to force the fighters into iran. 
