A poster of Syrian President Bashar al Assad hangs at the Wafideen camp in Damascus, Syria, on April 8, 2018.Omar Sanadiki / Reuters

Nearly one year ago exactly, American cruise missiles rained down on a Syrian airbase from which a deadly chemical attack had been launched against defenseless civilians. Now, one year later, mass murder and military reprisal appear again as exclamation points in a seven-year horror story of civilian suffering, state failure, and diplomatic malpractice. Once again, an American president has drawn a line and watched a brutal mass murderer cross it. Now he ponders what to do, knowing that what happens in Syria never stays there—that the conflict and its repercussions simply will not be contained.

If initial reports prove accurate, Bashar al-Assad, the president of Syria, has again used highly toxic chemicals to terrify and kill civilians. The Trump administration had hoped that its missile strike on Shayrat Airbase in April 2017 in response to the chemical attack launched from that base would deter such outrages. But deterrence was undone by Syrian cynicism and American thoughtlessness. Today, Trump said that his administration will make “some major decisions” on Syria over the next 24 to 48 hours.

The 2017 Assad regime assault on Khan Sheikhun, which prompted the American response, had featured the nerve agent sarin, one of the most toxic of weaponized chemicals. The same compound was used to kill civilians in August 2013, when Assad demonstrated to the world his contempt for President Barack Obama and Obama’s declared chemical warfare “red line.”

In 2013, Assad escaped punitive strikes by the United States by agreeing to a Russian-brokered deal that was supposed to remove his chemical munitions and block his ability to reconstitute a chemical-warfare capability. It did not do so, and nearly four years later Assad—perhaps led to believe by Trump’s remarks on the campaign trail that U.S. forces would not remain in Syria for long—returned to the use of sarin. No doubt he was shocked by the quick and lethal response.

Shock, however, did not translate into paralysis. With the support of Russia and Iran, Assad calculated that he could persist with his political survival strategy—mass homicide—and even employ chemical weapons for terror effects provided he stopped short of using sarin, which is banned under the 1993 Chemical Weapons Convention. Assad's addiction to chemicals was far from cured: His use of chlorine gas canisters (often packed in barrel bombs) never ceased. But he calculated that sarin was the specific red line not to be crossed.  

This calculation was not a hunch. Assad was able, with the help of Vladimir Putin, the president of Russia, to deduce soon after the cruise-missile strike of April 2017 that it was a one-time gesture—that, provided he stopped short of using sarin, any other acts of mass murder or state terror would draw no U.S. response beyond the customary hand-wringing in the United Nations Security Council.

What the Trump administration failed to do in April 2017 was deliver the following message to the Kremlin: You need to get your client out of the business of mass homicide. If he persists, we will exact prices at times and in places of our choosing. This is not just about chemicals. It is about what your client is doing to our friends, our allies, and to us by providing recruiting fodder for Islamist extremists around the globe. Perhaps that’s the message intended by Trump’s warning of imminent action.

Absent such a message, Assad saw the red line on sarin as a green light to do  everything else. On the chemical front, he continued his promiscuous use of weaponized chlorine, as he had done during the Obama years. And when he heard Trump speak publicly about his desire to get out of Syria quickly, he may have concluded that he could again indulge his addiction to the stronger stuff.

As he did a year ago, however, Assad may have miscalculated. Trump’s call to pull troops out of Syria notwithstanding, administration officials—long before the latest incident—were troubled and perplexed by the breakdown of deterrence they thought they had purchased with the April 2017 retaliatory strike. H.R. McMaster, the recently replaced national-security adviser, spoke publicly about that failure during the February 2018 Munich Security Conference.

Now the president and John Bolton, his new national-security adviser, face the possibility that Assad, a crime family chieftain, has again placed American credibility on the line. Based on the latest reports, the Pentagon has likely prepared a menu of regime targets from which to select. Can any good come from a decision to strike?

If the objective is to unseat Assad, an operation far transcending a punitive strike would be required. But this is not the objective. Instead, the aim of hitting the regime and hitting it hard would be twofold: to uphold the credibility of the United States while preventing the kind of global destabilization inadvertently set in motion by the red line erasure of 2013, and to tell Assad that the free ride for mass murder has ended. As matters now stand, his slaughter of civilians is, beyond the cost of aviation fuel and munitions, completely risk-free. And his message to the United States and its allies is clear: You cannot touch me. Indeed, give me reconstruction money or I’ll send more refugees in your direction.

Russia, if it is willing to risk direct confrontation with the United States, can try to close the airspace over eastern Syria, where U.S.-led operations against the Islamic State continue. And it may take more than a single strike to derail a campaign of mass homicide that has produced a humanitarian abomination and undermined the security of the west.

The Trump administration inherited a Syria policy whose unintended consequences exacerbated a humanitarian disaster, undermined U.S. credibility, and placed allies and friends at risk. Now it is forced again to confront that which its predecessor sought unsuccessfully to avoid: What happens in Syria will not stay in Syria. As long as Assad is free to murder whomever he wants with whatever he chooses, Syria will hemorrhage human beings for as far as the eye can see, and Islamist extremists around the globe will use Assad's crimes to find recruits for terror operations inside Syria and beyond. And as long as the United States proclaims red lines and promises consequences but does nothing, it places its reputation on the line everywhere. Remaining passive is not risk-free. The world watches as an American administration sets a value on its word.

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