United Nations Secretary General Ban Ki Moon fears the taboo against chemical weapons use is eroding, after President Barack Obama did not attack the Assad regime in 2013.

“It is imperative that those responsible for the use of chemical weapons should be held accountable,” Moon told the Organization for the Prohibition of Chemical Weapons in a letter. Moon said the repeated use of the weapons in Syria risks chemical weapons being “normalized in this or any conflict, present or future.”

“We have been very clear to the Assad regime … that a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized,” Obama told reporters in August, 2012. A year later, definitive reports confirmed widespread chemical weapons use by the Assad regime against civilians, including sarin gas.

Despite pressure from his own advisors, the international community, NATO, and U.S. allies, Obama demurred on his “red line.” Obama instead opted for a Russian-brokered deal with the Assad regime, in which Assad would give up his chemical weapons arsenal. “I’m very proud of this moment,” Obama told Jeffery Goldberg of The Atlantic. “I believe ultimately it was the right decision to make,” Obama elaborated.

An Organization for the Prohibition of Chemical Weapons report in late August, 2016, found evidence to “indicate potentially undeclared chemical weapons-related activities.”

“It is now impossible to deny that the Syrian regime has repeatedly used industrial chlorine as a weapon against its own people,” White House spokesman Ned Price even acknowledged after the report,

“It is crucial to hold those who use or intend to use chemicals as weapons accountable for their acts, as it is fundamental to deter all those who continue to believe that there is something to be gained in the use of toxic chemicals as weapons,” a U.N. report detailing Assad’s chemical weapons use closed.

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