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

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Wait that’s illegal in warfare? I’m going into recruit training for USMC in march and one event is called The Gas Chamber. Bunch a dudes go into a small room with burning CS gas, take off their masks after a while, then go out. It’s to “build trust in our equipment”

Then again military doesn’t really follow constitutions etc once you sign away to become gov property so I guess it doesn’t matter lol. Glad to know I won’t have to deal with it if I ever see actual combat hopefully