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The use of gas in war has been sparse since treaties were enacted.

Turkey, Syria, Iraq... all on their own people.  Rhodesian war saw use and Nicaragua saw similar use, though not aerosolized so much but instead sewn into soil otherwise intended for agriculture. Rhodesia's bane was anthrax,  which is a strange substance straddling chemical and biological when it has been weaponized- as its encapsulated in a protective cell where it can "live" a long, long time or until its disturbed or wetted.  In central America the govt and the rebels destroyed fertile soil in effort to deny food from one vantage, and economic asset to the other.  I don’t recall what they used if I ever knew, but it was something that takes generations to neutralize. 

An aside- the Vuelta bajo in Cuba is the tobacco growing region.  Cuba has a govt appointed "master blender" for their cigar manufacturing.  They had a introduction of new labels event way back in the early 2ks, and this master blender was asked "who is growing the best crop right now?" And he answered "the padron plantation"... it was a bit shocking because the padron's are in Nicaragua. Turns out what we believed was Cuban puros cigars are blends with variants from other nations. Thats significant because it shows how they are getting their feet back under themselves from the destruction of wars... that master blender went on to say "when Nicaragua and Honduras recovers, their soil is better for growing than our own"..... to better understand that, though, this was post hurricane Andrew which really jacked them (cuba) up badly- destroying that industry infrastructure and with heavy infusion of runoff, and whatever comes along with it, into the soil.

NASA, if interested in colonizing Mars, should pay attention to how those guys make and then keep their soil fertile.