Donald Trump did not waste time censuring Damascus in April of 2017, following reports that 80 civilians were subsequently killed and another 400 injured in a chemical attack on the rebel-held town of Khan Sheikhoun. The president acted.

“Tonight, I ordered a targeted military stroke on the air base in Syria from where the chemical attack was launched,” Trump said. “Years of previous attempts at changing Assad’s behavior have all failed.” Trump proceeded to connect the global instability that has resulted from the metastatic Syrian civil war to the crimes against humanity executed by Assad and defended by the regime’s allies. “It is in this vital national security interest of the United States to prevent and deter the spread and use of deadly chemical weapons,” Trump concluded. He was correct; it would be a tragic shame if this administration proved they never meant a word of it.

The 59 Tomahawk cruise missiles that targeted aircraft, shelters, radar and air defense, ammunition bunkers, and fuel stores at the Al Shayrat airfield were dismissed as inconsequential by the Trump administration’s critics. They alleged that the strike was too small to have any lasting effect on Assad regime behavior, particularly since the administration informed Russia ahead of the strikes, which allowed for the evacuation of Syrian military personnel from the strike zone. But the targeted missile strikes did have the effect of deterring the Assad regime from using chemical weapons against civilians, and not just the Sarin that was used on Khan Sheikhoun; chlorine, too.

Amid reports that Syria was preparing another chemical attack on civilian populations, the White House confronted the threat early. “[If] Mr. Assad conducts another mass-murder attack using chemical weapons,” former White House Press Secretary Sean Spicer warned in June, “he and his military will pay a heavy price.” These threats worked. At least, for a time.

That time has passed. Today, surface-to-surface missiles containing chlorine gas are raining down on populous towns like Saraqeb in Idlib Province and the Damascus suburb of Eastern Ghouta. The United Nations is still investigating these attacks, but it has blamed Damascus for similar chlorine strikes on civilians in the recent past. Despite disappearing from the headlines in the West, the situation in Syria is growing worse by the day. The fighting is flaring up again, and civilians are suffering. “Humanitarian diplomacy seems to be totally impotent, we’re getting nowhere,” United Nations humanitarian adviser Jan Egeland said last week. He added that Assad’s forces had prevented UN humanitarian relief missions from accessing certain besieged areas, and aid convoys have not reached some parts of Syria in two months.

The Trump administration now faces a moment of truth. It could preserve the moral authority it purchased after declining to merely scold the Syrian regime for deploying weapons of mass destruction against civilians, or it could retreat into a defensive crouch and act like the Syrian regime’s de facto defense counsel. That, to their everlasting shame, was the Obama administration’s approach to the use of chlorine munitions in Syria.

At a May 2015 press conference in which he defended his administration’s efforts to secure a deal to stall Iran’s nuclear weapons development, Barack Obama was asked about reports of over 30 chlorine bombings in Syria in the space of just two months. His response was devoid of moral clarity. “We have seen reports about the use of chlorine in bombs that have the effect of chemical weapons,” Obama told reporters. “Chlorine itself is not listed as a chemical weapon, but when it is used in this fashion, can be considered a prohibited use of that particular chemical.” He went on to say that addressing the potential use of chemical weapons was a job for the international community and Russia, and added that the deal he made with Moscow that supposedly neutralized Assad’s chemical weapons stockpile was a wild success. “Those have been eliminated,” Obama insisted.

Chlorine is a dual-use chemical that has industrial applications and, as such, is not subject to the same global prohibition that nerve agents like Sarin and VX are. But the Organisation for the Prohibition of Chemical Weapons lists chlorine as a choking agent with potentially lethal battlefield applications. This revelation should surprise no one who has a cursory familiarity with the events of World War I.

The last administration’s efforts to downplay the severity of chlorine attacks in Syria were grotesque. Obama’s appeal to Russia as a source of relief for the people of Syria—a nation that now actively blocks the international community’s efforts to extend the mandate of chemical weapons inspectors in Syria—was craven.

The last administration wanted to avoid the demands that history made on it, and it was a disgrace. Will the Trump administration abandon the course correction it embarked upon last April? Will it retreat to the same obtuse legalisms to which Obama appealed, even as the worst humanitarian and military crisis of this century intensifies? Will this president shirk his duty to humanity and to history, too?