The use of tear gas in warfare is banned under the Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on Their Destruction - which means the scenes we are watching unfold wouldn't happen in a warzone because it's banned under international law.

http://www.opcw.org/chemical-weapo…

It is legal to use domestically for riot control - but it says something to me that it's banned for use in war.