President Trump launched a cruise missile strike Thursday to punish the Syrian regime of Bashar al-Assad for the alleged use of chemical weapons. Assad’s apparent sarin attack represented yet another blow to the global norm prohibiting the use of chemical weapons. But the significant violation of the taboo in Syria is not likely to lead to the routine use of chemical weapons in future conflicts.

Syria represents the only open challenge to the norm against chemical weapons in more than 25 years. In the spring of 2013, the United States and France publicly alleged that sarin gas has been used in Syria. A June 2013 White House statement claimed that “the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale against the opposition multiple times in the last year.” Assad might have taken the international non-response as a green light for larger attacks — namely, the large-scale sarin attack of Aug. 21, 2013, in Ghouta, launched just days after a U.N. team arrived in Syria to investigate reports of previous attacks.

It was easy to rest with the conclusion that the global norm against chemical weapons was weakened by these violations. Many believe it was shattered when President Barack Obama did not bomb the Syrian regime in 2013. But a deal, made with Syria’s ally Russia, removed far more of the lethal chemical weapons than any military attack would have.

What does the U.S. strike against Syria mean for Bashar al-Assad and the region (Louisa Loveluck, Jason Aldag/The Washington Post)

This deal followed Obama’s initial response to the 2013 Ghouta attack, which was as pointed an exercise of norm enforcement as could be imagined. At the time, Obama said that “the United States should take military action against Syrian regime targets. … What message will we send if a dictator can gas hundreds of children to death in plain sight and pay no price? What’s the purpose of the international system that we’ve built if a prohibition on the use of chemical weapons … is not enforced?”

Obama appealed to the broader potential consequences if even this most minimally contested of norms could not be enforced: “Make no mistake — this has implications beyond chemical warfare. If we won’t enforce accountability in the face of this heinous act, what does it say about our resolve to stand up to others who flout fundamental international rules?”

Obama famously reconsidered military action, and sought congressional approval before launching the strikes. Secretary of State John F. Kerry indicated that to avoid an attack, the Assad regime could give up its chemical arsenal within a week. Russia seized upon the idea as the basis to avoid a military attack, and Syria agreed to join the Chemical Weapons Convention (CWC) and submit its facilities to inspection.

Less than a month after the 2013 attacks, Syria was on its way to allowing the Organization for the Prohibition of Chemical Weapons (OPCW) to verify the dismantling of its declared chemical weapons capability. The OPCW received the Nobel Peace Prize for its role, signaling the widespread global support for the enforcement of the taboo. All of this bolstered, rather than eroded, the chemical weapons norm.

Yet new challenges to the norm came after the agreement. Reports began to surface in 2014 of the use of chlorine bombs in Syria. A joint OPCW-United Nations investigatory mechanism in 2016 for the first time had the mandate to name names, which it did in reports to the U.N. Security Council concluding that the Syrian regime was responsible for at least three incidents and the Islamic State was responsible for one use of mustard gas.

Norms depend on punishment of those who violate them. Russia refused to accept the validity of these findings and wielded its veto power (often along with China) in the Security Council to shield Syria from a U.N. response of sanctions or force. The lack of a response to the first-ever violations of the CWC by a party was a serious failure to reinforce the taboo, and no doubt encouraged Assad to conclude that he could continue to use chlorine bombs. This week’s apparent escalation to sarin could well be the result of Assad feeling yet further emboldened after U.S. Secretary of State Rex Tillerson’s indicated last week that the “status of President Assad will be decided by the Syrian people.”

It is striking that nobody denies there should be a norm against the use of chemical weapons. No party ever claimed responsibility for using such weapons, while all accused — even the Islamic State — have vehemently denied such use and blamed others. This reinforces the idea that chemical weapons remain an aberration not to be accepted — in contrast to more vigorous challenges to other norms, such as against torture or those of the International Criminal Court in recent times. Despite worrisome allegations of chemical-weapons use by Sudan and the killing by VX of the North Korean leader’s half brother, with but four states not party to the CWC, there is not a stampede of would-be violators waiting in the wings to use chemical weapons.

Even granting additional possible motives, it is a remarkable testimony to the power of the taboo that Trump echoed Obama’s language when describing this week’s attack as an “affront to humanity” that “crosses many, many lines, beyond a red line” and that he, of all people, was moved to order a missile attack to reinforce the norm. Tillerson used the same analysis explaining the U.S. strikes: “[I]t’s important to recognize that as Assad has continued to use chemical weapons in these attacks with no response — no response from the international community — that he, in effect, is normalizing the use of chemical weapons, which may then be adopted by others. So it’s important that some action be taken on behalf of the international community to make clear that the use of chemical weapons continues to be a violation of international norms.”

It is impossible to imagine more powerful evidence of the continued salience of a norm for another would-be violator to ponder.

Richard M. Price is a professor at the University of British Columbia and the author of “The Chemical Weapons Taboo.”