What prompted the U.S. to act now, six years after the start of the civil war? The Washington Post's Amanda Erickson explains President Trump's decision to strike Syria. (Amanda Erickson,Jason Aldag/The Washington Post)

The Trump administration’s effort to underscore its certainty that Syria carried out a chemical weapons attack has demonstrated a potential new U.S. policy: The use of nerve agents such as sarin will prompt a military response, even if it’s less certain that unleashing other chemical weapons, such as chlorine, will.

The April 4 attack in the Syrian town of Khan Sheikhoun killed scores of men, women and children, and deeply upset the president after he saw images of the aftermath, he said last week. Senior U.S. officials began preparing a military response by the next day, and dozens of Tomahawk missiles launched from Navy destroyers slammed into the Shayrat air base in Syria about 3:30 a.m. Friday local time.

Defense Secretary Jim Mattis said Tuesday that the response was “not a harbinger of some change in our military campaign” in Syria, which is focused on countering Islamic State militants, but something that needed to be done because Syrian President Bashar al-Assad had not followed through on a 2013 United Nations agreement he reached to give up specific chemical weapons.

“The strike that we’re talking about here today was directed at the people who planned it, who held onto the weapons contrary to what they had promised the international community and United Nations when they said they had gotten rid of those weapons,” Mattis said in his first Pentagon news conference since taking over the Defense Department. “And the reason for the strike was that alone.”

Defense Secretary Jim Mattis on April 11 said Syrian President Bashar al-Assad "should think long and hard" about using chemical weapons again on civilians. (Reuters)

Trump has decried the use of both barrel bombs and chemical weapons in recent days, but the Assad regime has used them and conventional bombs for six years in the Syrian civil war, killing tens of thousands of people each year. The administration of President Barack Obama threatened to carry out strikes against Assad in 2013 after his regime carried out an attack with the nerve agent sarin that killed 1,429 people, according to a White House estimate. Obama ultimately stopped after the Assad regime agreed to turn over its chemical weapons.

Nerve agents such as VX and sarin are among the most deadly chemical weapons, and are subject to the agreement Syria reached with the Organization for the Prohibition of Weapons in 2013. Other chemicals such as fluorine and chlorine are considered dangerous, but are not typically as deadly and can be used in industry. That has allowed the regime to continue possessing them, and created what some analysts call a loophole in the agreement.

The Assad regime has launched attacks with industrial chemicals several times since 2013, including at least twice during the Trump administration, U.S. officials said. Senior military officials said last week at the Pentagon that attacks occurred March 25 and March 30, leading up to the more serious attack with a nerve agent April 4.

Neither Obama nor Trump authorized a military strike until last week, after the first documented use of a nerve agent by the Syrian regime in an attack since 2013. Senior military officials, speaking on the condition of anonymity to discuss sensitive information, have referred to the chemical used April 4 as “a nerve agent like sarin,” and administration officials went further and specifically labeled it as sarin.

Mattis, asked Tuesday during a news conference whether he viewed the use of barrel bombs carrying chlorine differently from sarin, deflected the question and referred back to the attack last week, saying that he wanted to “say very clearly” that if Syria used chemical weapons that violated the Geneva Convention that guides warfare internationally and the agreement the Assad regime signed in 2013, “they are going to pay a very, very stiff price.”

Asked after the news conference how chlorine could best be defined, Mattis said “I really don’t want to.” The Syrian government signed the Geneva Protocol banning the use of chemical weapons in 1968, but the use of chlorine in Syria has not yet been stopped.

A senior defense official, speaking on the condition of anonymity to describe sensitive information, said that chlorine has a different status than sarin under international law, but Mattis does not want to say what will happen if Assad continues to use it. The idea, the official said, is to give the regime pause before using any kind of chemical weapon.

During the news conference, Mattis said the United States will need to decide as a matter of policy how it will respond in the future to the use of any kind of chemical weapon, including chlorine, in Syria.

“There is a limit, I think, to what we can do,” Mattis said. “And when you look at what happened with this chemical attack, we knew that we could not stand passive on this.”

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