Hundreds of victims streamed into Damascus-area hospitals last week, all suffering from the same symptoms. They trembled. They had blurred vision. They gasped for breath. They convulsed.

And then many died.

Those are the classic signs of chemical weapons poisoning, and they provide a graphic reminder of why these horrific weapons of mass destruction were banned after World War I by the 1925 Geneva Protocol.

“Only 4 percent of all battlefield deaths in the Great War had been caused by gas, yet the foul nature of those deaths meant that gas held a particular terror in the public imagination,” writes historian Andrew Roberts in a Wall Street Journal op-ed. “Since 1925, it has only been countries that are recognized to be outside the bounds of civilization that have taken recourse to it.”

Those include Saddam Hussein’s Iraq in 1987 and 1988 and Benito Mussolini’s Italy in a campaign against Ethiopia from 1935 to 1941.

The U.S. moved closer Monday to a declaration that President Bashar Assad’s Syria joined that shameful list last week.

On Monday, Secretary of State John Kerry said that the use of chemical weapons in Syria was “undeniable.”

“Moreover, we know that the Syrian regime maintains custody of these chemical weapons. We know that the Syrian regime has the capacity to do this with rockets. We know that the regime has been determined to clear the opposition from those very places where the attacks took place. And with our own eyes, we have all of us become witnesses,” Kerry said.

President Barack Obama has warned Assad that if he “made the tragic mistake” of deploying chemical weapons, “there will be consequences and you will be held accountable.”

There are strong indications that America and its allies are preparing to launch military action against Assad. Such action may not be imminent. Kerry said the U.S. is assessing more information about the chemical weapons attack and will release that information “in the days ahead.”

If a response is launched, one possibility is the use of cruise missiles against the heart of Assad’s forces. Any response will be most effective if mounted by a large coalition of NATO allies and Arab nations.

Realistically, there are limits to the damage the U.S. and its allies can inflict from airstrikes alone. Such attacks may weaken Assad and help the rebels regain lost ground. They could give the rebels enough leverage to push Assad to the negotiating table.

They would, without doubt, demonstrate that the use of chemical weapons invites a punishing response.

This is vital. A sharp military retaliation by the U.S. and its allies will show not only the Syrian strongman but other dictators around the globe that they cannot deploy such terrible weapons with impunity. That anyone who dares use these weapons will pay a steep price. That the world will not shrug and look away.

The war in Syria has killed more than 100,000 people with conventional weapons and sent more than a million people fleeing from their homes. Many Americans may ask, if the U.S. did not intervene to stop those deaths, why should it act now?

Because the use of chemical weapons is a red line drawn not just by the U.S. but by the entire world, to protect civilians. It may seem like a strange distinction: Someone killed by a conventional rifle or bomb is just as dead as someone killed by a chemical weapon.

But the damage from chemical agents, like nuclear weapons, cannot be finely targeted. Such weapons can kill wide swaths of people in a matter of seconds or minutes. They pose a special risk to civilians. These are weapons that many governments possess but few ever imagine using.

The 1925 Geneva Protocol said the prohibition against chemical weapons “shall be universally accepted as part of International Law, binding alike the conscience and the practice of nations.” Note that phrase: The conscience and the practice of nations. For nearly a century, nearly every government has abided by this treaty, forswearing use of such weapons. Were that to change, should such weapons be regularly deployed, wars across the globe would be even more destructive and ruinous.

Assad may have calculated that he will not be caught or punished for a war crime. That he can win his civil war by unleashing ever-escalating horrors upon innocent civilians, including women and children. The world needs to show him that he is wrong.

This editorial appeared in the Chicago Tribune on Tuesday, Aug. 27.