Defense Secretary James Mattis on Thursday stressed that the use of chemical weapons by Syria’s government would warrant a response from the United States.

At his first press conference as the Pentagon’s chief — and in his first public comments since the U.S. launched 59 Tomahawk missiles at a Syrian airfield last week — Mattis made it clear that the Pentagon believes Syria’s government was behind a chemical weapons attack earlier this month in Iblid Province that left 80 civilians dead.

Syria under President Bashar Assad has been embroiled in a yearslong civil war.

“This military action demonstrates the United States will not passively stand by while Assad blithely ignores international law and employs chemical weapons he has declared destroyed,” he said.

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“The Syrian regime should think long and hard before it again acts so recklessly in violations to international law against the use of chemical weapon.”

Mattis warned that the future use of chemical weapons by Assad risked another U.S. response — though he suggested this would only be for chemical weapons listed under an international agreement.

“The use of chemical weapons — contrary to the Geneva Convention that Syria signed up for, using chemical weapons that Syria agreed under UN pressure to remove from their arsenal. ... They are going to pay a very, very stiff price,” Mattis told reporters at the Pentagon.



The comments about the Geneva Convention are important because Syria has used chlorine in barrel bombs launched at civilians in the last year.

Chlorine is not included in the Geneva Convention, which banned the use of chemical weapons on the battlefield after World War I.

The attack that hit Idlib Province was believed to have used the sarin gas, which is included in the Geneva Convention.

Mattis declined to elaborate when he was asked repeated questions about whether a chlorine-based attack might draw a U.S. response.

He received several questions on whether chlorine falls under the chemical weapons category after he remarked there had been several instances of chlorine used on civilians.

“I really don't want to clear it up right now,” he told reporters after the briefing.



Instead, Mattis asserted that the retaliation strike was a “policy decision by the United States.”



“There is a limit I think to what we can do and when you look at what happened with this chemical attack we knew we could not stand passive on this,” he said. “The intent was to stop the cycle of violence into an area that even in World War II chemical weapons were not used on the battlefield. ... To stand ideally by when that convention is violated that is what we had to take action on urgently in our own vital interests.



“Chemical weapons are chemical weapons so that is the issue if you're taking about the strike we took,” Mattis said. “It's not about whether it was delivered by a helicopter with a barrel bomb ... or a fighter aircraft with a bomb, it's about chemical weapons and we've made clear where we stand.”



Mattis also stopped short of calling any major attack on civilians as a red line for the United States.

“I don't believe I've talked about red lines, I generally shy away from them myself. I recommend Assad be rather cautious violating international law with chemical weapons, that could be considered a red line to some people,” he said.