When President Donald Trump announced on Friday night that he had ordered airstrikes on Syria, he explained the decision with an appeal to a longstanding international consensus that the use of chemical weapons — such as were used a week ago on Syrian opposition forces and civilians in Douma, per the assessment of the U.S., despite denials from the Syrian regime — is a wrong that merits such a forceful response. France’s President Emmanuel Macron and Britain’s Prime Minister Theresa May have made it clear that they essentially feel the same.

“Following the horrors of World War I a century ago, civilized nations joined together to ban chemical warfare,” Trump noted in his speech, later adding that the world never wants to see the “ghastly specter” of those weapons return.

The indications that such an announcement might be coming had been present for days since the initial report of the attack, which left Trump once again considering how such news might change his plans, as he had formerly expressed a desire to get the U.S. out of Syria.

The news follows a similar pattern to the events of about a year ago, when the use of chemical weapons in Syria and the resulting horrific imagery of the dead seemed enough to prompt a reversal in President Trump’s stated aversion to U.S. intervention there.

And yet noncombatants are killed in the course of war in countless other fashions, and attempting to rank the cruelest ways to kill is an exercise in both futility and inhumanity. So why is it that chemical weapons spark such automatic revulsion when years of bombs and bullets have not? Though Trump correctly pinpointed World War I as a turning point, in fact the story is even older than that.

Early Developments

Chemical and biological weapons predate even our modern understanding of what those terms mean; there’s evidence of the use of noxious gas as a weapon in the ancient world and, as TIME has pointed out, the purposeful spread of smallpox in the 18th century as a tactic against American Indians was its own kind of biological warfare.

The feeling that such poisons are not an appropriate or honorable tactic of warfare is also an old one. As Julian Perry Robinson has written in his history of the subject, the earliest example of that idea might possibly be found in ancient Indian epics. And in Greek mythology, the use of poison as a weapon of war was often considered cowardly, a tricky technique used by those who were not heroes. By 1675, according to the Organization for the Prohibition of Chemical Weapons, France and Germany had signed what was the first international treaty that limited such weapons (in this case, poisoned bullets).

So it should be no surprise that in 1899, during a conference at the Hague on the laws of war, it was decided by the signatory nations that it would be “especially” prohibited “to employ poison or poisoned arms.” Though the convention they signed did not come with any enforcement mechanism, the moral imperative to avoid using poison during wartime had been established by that point.

But, during the 19th century, the technology that could be deployed in the use of chemical weapons underwent a shift. Pressurized canisters of noxious gas could replace the poisoned arrow tips of the past. As Robinson describes, Germany had an “industrial edge” in the development of such technology, and soon enough a reason to want to take advantage of that edge. The infamous trench-based stalemates that quickly set in after the 1914 beginning of World War I made the combatants eager to try something new — and that something included gas, even despite those centuries of history condemning such an idea.

Read More: These Battlefields Were Devastated in World War I. See What They Look Like Today

The World Wars

On April 22, 1915, “German infantrymen gave the world its first whiff of poison-gas warfare,” as TIME later recalled, “by sending a huge, grey-green cloud of noxious chlorine rolling over two French divisions in the trenches at Ypres, killing 5,000, incapacitating 10,000, and cutting a 31-mile swath in Allied lines.” Chlorine, phosgene, chloropicrin and mustard gas were all used during the Great War, though all involved — both sides developed and used chemical weapons — discovered that gas could be a difficult tool to use well, as the wind had to cooperate to make it work and developments such as increased use of tanks mitigated some of the chemicals’ impacts.

By TIME’s later estimate, that war saw 124,000 tons of chemical weapons used, between both sides, and the resulting deaths of 91,000 soldiers.

So it was that, in the wake of that conflict, the Treaty of Versailles included a ban on Germany manufacturing any poison gas. In addition, as members of League of Nations met in Geneva in 1925, delegates from 45 countries sought to find a out a way to come up with what TIME called back then “a protocol generally prohibiting chemical and bacteriological warfare.”

“[The] use in war of asphyxiating, poisonous or other gases, and of all analogous liquids, materials or devices, has been justly condemned by the general opinion of the civilized world,” the protocol proclaimed. The Geneva Protocol, however, did not end the history of chemical weapons. It had “no teeth,” per TIME, and allowed nations to make and keep such agents as long as they promised not to use them. (The U.S., which had not actually ratified its membership in the League, did not sign the Geneva Protocol at the time.)

And, before the League of Nations participants would have hoped, the world had cause to put the Protocol to the test. The Great War turned out not to be, as had been hoped, the “war to end all wars.”

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When World War II began, as TIME noted in 1944, “the horror and repugnance aroused by the use of gas in World War I [was] still alive,” though some Americans argued that using chemical weapons would be a more modern and humane approach to warfare. Yet, though soldiers had gotten much better prepared to withstand a gas attack since the 1915 Ypres battle that was still so fresh in humanity’s collective memory, chemical weapons still held the deadly threat of death for civilians. Nor was it necessary to use chemical weapons, one military official told the magazine: in order to really make a difference the gas must be used by a combatant with a superior air force, and having a superior air force meant there was no reason to use gas.

In 1943, President Franklin Roosevelt warned the Axis powers not to use poison gas during war, and promised that the U.S. would only do so in retaliation:

Use of such weapons has been outlawed by the general opinion of civilized mankind. This country has not used them, and I hope that we never will be compelled to use them. I state categorically that we shall under no circumstances resort to the use of such weapons unless they are first used by our enemies. As President of the United States and as Commander in Chief of the American armed forces, I want to make clear beyond all doubt to any of our enemies contemplating a resort to such desperate and barbarous methods that acts of this nature committed against any one of the United Nations will be regarded as having been committed against the United States itself and will be treated accordingly. We promise to any perpetrators of such crimes full and swift retaliation in kind and I feel obliged now to warn the Axis armies and the Axis peoples, in Europe and in Asia, that the terrible consequences of any use of these inhumane methods on their part will be brought down swiftly and surely upon their own heads. Any use of gas by any Axis power, therefore, will immediately be followed by the fullest possible retaliation upon munition centers, seaports, and other military objectives throughout the whole extent of the territory of such Axis country.

Even as Nazi scientists were experimenting with new and potent poison gases, including nerve agents like sarin, some of which were manufactured by concentration camp prisoners, Hitler chose not to endorse their use against Allied soldiers. (“Although many senior military officers encouraged Hitler to deploy their powerful new chemical weapon, he waffled, likely for two reasons,” according to Sarah Everts’ summary of the history for the Chemical Heritage Foundation. Those reasons were Hitler’s own experience as a victim of battlefield gas in World War I and his worry that the Allies might have developed similar technology and would retaliate in kind.)

But despite what happened on the battlefield, deadly chemicals were widely used by the Nazi regime as part of their effort to wipe out the population of Jews and other minorities from the area they controlled. For example, in 1944, when TIME’s Moscow Correspondent Richard Lauterbach visited the Majdanek concentration camp and reported on the tour he received from Dmitri Kudriavtsev, Secretary of the Soviet Atrocities Commission, he saw the bathhouses in which Nazis had used Zyklon B to kill as many as 250 people at a time. He was told that on Nov. 3, 1943, alone, “they annihilated 18,000 people — Poles, Jews, political prisoners and war prisoners” with the gas.

New Agreements, New Trespasses

After World War II, as proud as the world powers may have been to have avoided the use of chemical weapons during the battles, the Cold War arms race extended to chemical-weapons research competition, too. One rationalization for that research was helped along by the widespread Cold War idea that, as long as one didn’t strike first, any weapon could be on the table. Plus, the Geneva Protocol only prevented the use of those weapons, not the creation or storage of them.

So it should perhaps be no surprise that the late 20th century was a time of both new diplomatic advances and new uses of new weapons, despite those advances.

On the one hand, the U.S. — after decades of supporting the doctrine voiced by Roosevelt — signed agreements that barred the use of chemical weapons, as did other nations. On the other hand, research continued apace, and the prohibition on the use of chemical weapons did not completely stop them from being put into action.

Controversy over whether the U.S. was using gas in Vietnam raged during the 1960s — that the U.S. was using chemicals such as Agent Orange was clear, but whether that herbicide counted as a chemical weapon as governed by the Geneva Protocol was disputed — and by the end of that decade the U.S. was spending hundreds of millions of dollars a year on its chemical and biological weapons program, stockpiling the results. Then, in the wake of that controversy, President Nixon resubmitted the Geneva Convention to the U.S. Senate, shortly before the 1972 Biological Weapons Convention — which strengthened the biological-weapons portion of the Geneva protocol — was submitted, too. Both were approved by the Senate in late 1974 and ratified by President Ford in early 1975.

In 1988, Iraq used poison gas on Kurdish towns during the course of its war with Iran. “The bloated bodies of Kurdish residents littered the silent streets of the northern Iraqi town of Halabja. A dead turbaned man who had tried to shield a porcelain-faced infant in his arms from a cloud of poison gas lay frozen in time on a road. Families died together in their homes or in cars. The dead were among the hundreds and possibly thousands of victims of one of the worst chemical-warfare attacks since World War I,” TIME reported. The weapons used were primarily mustard gas. The world’s response was somewhat muted — perhaps due to a lack of desire to take the side of Iran among many of the Geneva Protocol signatories who might have otherwise spoken up — but President Ronald Reagan said in a speech that year that the use of those weapons “jeopardizes the moral and legal strictures that have held those weapons in check since World War I.”

In 1993, after the fall of the Soviet Union, an even tougher U.N. Chemical Weapons Convention was created. This agreement prohibited stockpiling. At the time of the signing, both the U.S. and Russia had tens of thousands of metric tons of chemical weapons in store. Following through on the timely destruction of the chemical agents, as required by that agreement, has proved difficult.

And yet one question, raised by Julian Perry Robinson, has remained unanswered through these many decades: if chemical weapons were more tactically useful, easier to control and harder for armies (if not civilians) to guard against, would those who make such decisions have come up with justifications for their use?

After all, as a TIME essay in 1969 began, “The dark side of progress is man’s spectacular skill at devising better and better ways to kill other men.”

Write to Lily Rothman at lily.rothman@time.com.