This wasn't some homemade chemical attack. It was proper chemical weaponary using Sarin gas.
Even though Asaad is winning his army is battle weary and he has started drafting in the Secret Police to help fight so it's still a slog. Perhaps because he feels he has the backing of Russian he felt emboldened to use chemical weaponary because it is an easier way if clearing an area that having to send in troops.
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