During WWII the USN had at least one shipful of mustard gas as part of the invasion for mainland Italy. It was intended for use against the Germans, should they have used gas first.
Since ISIS has chosen to deploy chemical weapons — retaliation with same is called for today.
When the Germans attacked Bari, also known as “Little Pearl Harbor”, one of the ships they bombed carried mustard gas, and it killed a lot of allied troops.
That attack was kept secret for decades.
And that ship full of mustard gas was sunk in an Italian harbor by German bombers, causing US casualties.