As far as US Person knows the United States has not crossed its supposed 'red line' and used chemical weapons directly against enemies in the field. However, it has used chemical weapons, most extensively in Vietnam. A mixture of chemical defoliates code-named Agent Orange contained possibly the most carcinogenic chemical known to man, dioxin. Nearly two million gallons was sprayed on the rainforests and crop lands of Vietnam with the delusional intention of depriving the Viet Cong of their jungle refuges and food crops*. With typical macabre bravado of the era, participants of the aerial spraying operations (Operation Ranch Hand) from 1967 to 1970 adopted the motto, "Only you can prevent forests!". An estimated 39,000 square miles of agricultural land was destroyed, and about 20% of Vietnam's forests were sprayed at least once over a nine year period. Without a doubt, "Operation Trail Dust", the US chemical warfare program in Southeast Asia, is a war crime as this graphic Australian video documents:
The Vietnamese victims lawsuit against American chemical companies that manufactured the chemical defoliates was dismissed and the dismissal upheld by the US Second Circuit Court of Appeals in June, 2007. The Appeals Court ruled that despite the herbicides containing a known poison, they were not intended to be used against humans, therefore were not chemical weapons under international law. The Supreme Court denied certiorari.
*RAND Corporation Memo 5446-ISA/ARPA: "the fact that the VC [Viet Cong] obtain most of their food from the neutral rural population dictates the destruction of civilian crops....it will be necessary to destroy large portions of the rural economy--probably 50% or more." The assertion that the rural population is neutral was wishful thinking. In order to save the country from communism, it was necessary to destroy most of it. The Geneva Conventions and protocols thereto prohibit waging war against civilian populations, and using chemical or biological agents to destroy plants in a time of war.