Why is illegal for CS gas (tear gas) to be used in warfare but countries can use it on their domestic population?

Question

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In: 4

Answer 1

For whatever reason, you’re a country at war with another country.

You have a stockpile of chemical weapons, but you don’t use them, because 1, fuck that stuff and 2, it’s illegal. Your enemy also have chemical weapons, like mustard gas and sarin.

Now, you have reports coming in that the enemy shot cannisters at your troops that released a chemical, and immediately your troops reported trouble breathing, blindness, pain, etc…

What do you do? Wait until reports come in that clarify it was just tear gas and not mustard or sarin? Or do you respond in kind and launch your chemical weapons?