Despite their fearsome reputation, most weapons of mass destruction — even the conventional ones — are not used often. Like a vaccine, their first use seems to inoculate the world against further uses. But in Syria — struck by Western missiles early Saturday — the inoculation effect against poison gas has been failing. And so is the U.S. policy that contributed to it.

Looking at history, the pattern is striking: The first major use tends to be the last one, or at least the biggest one. Atomic bombs were used to end World War II. That war saw the only large-scale conventional air raids on cities — first by Nazi Germany, and then the Anglo-American powers.

Biological weapons have been absent from the battlefield. So far, anthrax has been used only in the terror attacks in the United States after 9/11. Even poison gas, the easiest WMD to make, was used more in World War I than since.

The exceptions to this rule are telling. Benito Mussolini used poison gas in Ethiopia in 1935, but tried to conceal it. The Japanese cult Aum Shinrikyo used sarin gas in terror attacks in the 1990s. And, of course, Adolf Hitler used gas to kill millions of Jews, and other victims, in the Holocaust.

That terrible crime illustrates what gas became after World War I: a weapon used in secret against innocent civilians, and usually against people whom the poisoner deemed subhuman. It was not a weapon of the battlefield, used between soldiers in regular, professional militaries.

There’s no single reason why it was this way. One is that gas has not been an effective weapon. Germany first used chlorine gas in 1915 against French troops, who broke. But the Australian forces who were attacked next stood their ground.

Another reason is that atomic bombs, large conventional bombers and biological weapons are difficult to make and not easy to deploy. Not every nation or terrorist group can summon the resources.

Then there’s deterrence. The key reason nuclear weapons have never been used is simple: the threat of retaliation in kind. And there’s one more reason. War is not mere violence; it is a human invention. Unlimited violence is naturally repugnant. It is murder, not war. WMDs offend this instinct. Only murderers like Hitler, or terrorists like Aum Shinrikyo, lack this sense.

Bashar Assad regime’s use of chlorine gas marks it as murderous, but that’s not news. He’s been using gas, with Russian connivance, since 2013. Even worse, it’s working. Assad is winning — in fact, he’s won.

What’s more troubling is that the inoculation effect has failed. Fear of publicity has been no deterrent. Assad has been able to bring the necessary resources to bear. Gas still isn’t a war winner, but he has found that, in a civil conflict, it’s a useful tool to cow potential rebels.

And, of course, Assad’s use of gas provoked no deterrent response. President Barack Obama drew a red line in 2013 and failed to uphold it. To his credit, President Donald Trump launched a retaliatory strike in 2017, but that single attack did not have the desired effect.

But what’s at stake goes beyond Syria. The best way to prevent epidemics is for everyone to be inoculated. If they’re not, diseases spread. The United States recognizes this. Since 1945, it has consistently opposed the proliferation of WMDs.

But the more Assad uses gas, the more everyone sees gas as a viable weapon. The fading of the inoculation effect in Syria gives dictators facing rebellions an option that used to be unthinkable: Use gas. That’s not just bad for humanity or for Syria. It’s a defeat for the United States.