As predicted, Islamic State set fire to a Mishraq chemical plant and sulphur mine 30km south of Mosul last week as they fled. The toxic cloud, which includes deadly sulphur dioxide and hydrogen sulphide combined with residue from burning oil wells, is a fatal cocktail for those caught in the open or without gas masks.The cloud currently stretches 20km to 30km to the south over sparsely populated ground, but if the wind changes to the east, there is a slim chance that the Kurdish capital Erbil, 60km away, could come under threat.Mishraq is the latest step in IS's plan to defend Mosul at all costs. It is no surprise that the ultimate terror organisation is looking to the ultimate terror weapon, chemicals, to save its "caliphate HQ".Chemical weapons have kept Syrian President Bashar al-Assad in power a number of times in the past three years, and they are highly effective in defending built-up areas and cities.The IS chemical weapons programme is being run by Iraqi dictator Saddam Hussein's former Baathist scientists and experts recruited globally.They are making mustard agent (gas) in Mosul and have an almost limitless amount of chlorine to fill into mortars, which they have used to attack the Peshmerga for the last 12 months, and scores of improvised explosive devices. ..