The USA does not tolerate chemical weapons?

The USA used nuclear bombs against two civilian cities in Japan, firebombed surrendered German soldiers after the war was over, uses depleted uranium on country-wide murdering fields, used chemical weapons in Vietnam on the scale of the whole country, supports chemical-weapon attacks against guerilla resister-supporting populations in Latin America, allows Israel to use white phosphorous against the Gazan population, ... and used economic sanctions (blockade) to kill a million civilians in Iraq. 

These are just the incidents that come immediately to mind.

And now, the USA has such a high intolerance of chemical weapons that is does not even need evidence of their use before it bombs and sanctions a nation to death.