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Why We Didn’t Use Poison Gas in World War II

May 2024
14min read

In a conflict that saw saturation bombing, Auschwitz, and the atom bomb, poison gas was never used in the field. What prevented it?

Forty years ago, on August 6 and 9, 1945, American B-29s dropped two atomic bombs on Japan, killing at least 110,000 and possibly 250,000 Japanese and speeding that nation’s surrender. During four years of bitter fighting, World War II had become for the United States virtually total war, in which morality had slowly been redefined to allow the intentional bombing of civilians.

Ever since, however, use of these atomic weapons has raised troubling questions about American ethics during the war. Yet lost in the concern is a related question: Why didn’t the United States also initiate gas warfare? Did an older sense of morality, rooted in the decades before Pearl Harbor, bar this form of war even as other moral constraints eroded?

During World War II, international law did not actually bar the United States from using gas warfare—although America had signed the 1925 Geneva Protocol outlawing gas, the Senate had never ratified it. Yet every peacetime President from Warren G. Harding to Franklin D. Roosevelt had defined gas as immoral and pledged to abide by the agreement. The cruel gas deaths of World War I, painfully etched in memory, constituted a powerful ethical deterrent. In a secret memo, written soon after Pearl Harbor, Secretary of State Cordell Hull, a Tennessee Democrat and proud Wilsonian, urged the administration to declare unilaterally that it would continue to observe this prohibition. Secretary of the Navy Frank Knox, a Chicago Republican, readily agreed: “The Navy is against the use [of gas] in wartime.”

But Secretary of War Henry L. Stimson, the leading Republican in the bipartisan war cabinet, opposed Hull’s proposal. Any public statement, Stimson contended, might provoke a domestic debate on moral and political issues that would delay military production of gas and lead Germany and Japan to view America as weak. Stressing that these enemies, as well as Italy, had repeatedly violated treaties, and claiming that Italy had used gas in Ethiopia and that Japan had done so in China, Stimson concluded that “the only deterrent is fear of our retaliation. I strongly believe that our most effective weapon on this subject at the present time is to keep our mouths tight shut.”

Events soon undercut this cautious strategy of silence. In May 1942 Prime Minister Winston Churchill, fearing German gas warfare against Russia, publicly warned Adolf Hitler that Britain would retaliate with gas on German cities. The next month President Roosevelt, citing new accusations against Japan, issued a similar warning: “If Japan persists in this inhuman form of warfare against China or against any other of the United Nations, such action will be regarded by this Government as though taken against the United States, and retaliation in kind and in full measure will be meted out.”

For Churchill, an ardent advocate of poison gas in World War I and never committed to the subsequent moral code against it, and for Roosevelt, sincerely committed to that code, the warnings were designed to deter enemies from launching gas warfare and thus making retaliation necessary. Roosevelt continued to receive reports of scattered incidents of Japanese gas warfare against China, but he and his advisers correctly interpreted these abuses as decisions made by local commanders, not as a statement of a new Japanese policy. Had the President been seeking a pretext to retaliate with gas, he might have seized upon these reports. But his caution and his moral inclinations reinforced each other, and he preferred instead to issue warnings and hope for the best.

In June 1943, using a State Department draft, Roosevelt sharply reaffirmed United States policy on gas warfare: “Use of such weapons has been outlawed by the general opinion of civilized mankind. This country has not used them, and I hope 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.” Such a ringing statement of morality, even as the barbarities of war chipped away at other parts of the international military code, made it unlikely that FDR would yield easily to entreaty or to the claim of exigency.

His known opposition to initiating gas warfare blocked some American military planners from seriously considering it and thus deterred the very bureaucratic actions that might have pressed him to reconsider his commitment. He also received indirect support from the Navy and Army Air Force. Top Navy officials had concluded that gas should not be used against civilian populations and that it was not especially effective against military targets. “Hit for hit and pound for pound,” said Adm. Ernest King, chief of naval operations, “no service chemical is considered to offer as great effectiveness as high explosive.” Air Force leaders, committed to aerial bombing, had reached a similar conclusion.

Even so, the budget and total personnel of the Army’s Chemical Warfare Service (CWS) soared. Held to an average annual appropriation of $1.5 million and to about five hundred Army personnel through the mid-thirties, in 1942 the CWS received one billion dollars and had more than sixty thousand employees. Its tasks included preparing for gas and bacteriological warfare, as well as producing incendiaries for bombing, flamethrowers, and other devices.

As the war continued, the Chemical Warfare Service chafed under FDR’s imposed restraints. In mid-December 1943, after the bloody Pacific Battle of Tarawa, which had cost the United States more than thirty-four hundred casualties in four days, Maj. Gen. William N. Porter, chief of the Chemical Warfare Service, pleaded with Army superiors to start using gas. In view of American air superiority, he argued, there would be no danger of Japanese reprisals. “We have an overwhelming advantage in the use of gas. Properly used gas could shorten the war in the Pacific and prevent loss of many American lives.”

He could find some popular support for his view. “We Should Gas Japan,” declared the New York Daily News, and the Washington Times Herald asserted, “We Should Have Used Gas at Tarawa” because “You Can Cook ’Em Better with Gas.” But such opinion was in the minority; about 75 percent of Americans still opposed initiating gas weapons.

Porter’s pleading proved unsuccessful within the Army—primarily for military, not moral, reasons. Maj. Gen. Thomas T. Handy, of the Army’s Operations Division, explained that the use of gas against Japan might provoke Germany “to gas in retaliation.” The war was, Handy argued, a two-theater struggle; likely advantages, no matter how attractive in the Pacific, would be outweighed by the likely disadvantages in Europe, the primary theater. “The difficulties inherent in amphibious operations [in the forthcoming D-day landing] against the continent are tremendous and no action should be initiated which would provide the Germans with an excuse for using gas as a defensive weapon against such operations.”

Shortly before the D-day invasion the British military chiefs began worrying that the decision by Gen. Dwight D. Elsenhower, Supreme Allied Commander, to use white phosphorus would violate the 1925 Geneva Protocol—which Britain, unlike the United States, was bound by—and might unleash German retaliatory gas attacks. “It is difficult,” the Ministry of Defense warned, “to draw a firm line between the use of white phosphorus for smoke and as an incendiary (which is legal) and its use primarily against personnel (which may be illegal).” Elsenhower refused to back down. By the time the issue percolated up to Churchill on June 21, the early assault on Normandy was over, and apparently the prime minister decided against appealing the matter to Roosevelt.

After the war an Army chemical-warfare expert concluded that the use of gas by Germany could have delayed the Allied cross-Channel attack by six months. “Such a delay,” he noted, “could have given the Germans sufficient time to complete the new V-weapons, which would have made the Allies’ task all the harder and England’s long range bombardment considerably worse.”

Furious at the V-I assault on Britain, Churchill wanted to “drench the cities of the Ruhr” with gas.

About a week after D-day, Germany launched a massive V-I assault upon Britain, killing twenty-seven hundred people, injuring ten thousand, and damaging the homes of more than two hundred thousand. Eager to punish Germany and hoping to deter future rocket attacks, Prime Minister Churchill wanted to “drench the cities of the Ruhr and many other cities in Germany [with gas] in such a way that most of the population would be requiring constant medical attention.” He informed his military advisers: “It is absurd to consider morality on this topic when everybody used it in the last war without a word of complaint from the moralists or the Church. On the other hand, in the last war the bombing of open cities was regarded as forbidden. Now everybody does it as a matter of course. It is simply a question of fashion changing as she does between long and short skirts for women.”

Recognizing that he was threatening to cross what many defined as a moral threshold, Churchill indicated that he would use gas only if “it [is] life or death for us, or [if] it would shorten the war by a year.”

His directive to military advisers was blunt and chilling: “I want a cold-blooded calculation made as to how it would pay us to use poison gas. … I want the matter studied in cold blood by sensible people and not by that particular set of psalm-singing uniformed defeatists which one runs across now here now there.”

British military advisers soon dashed his hopes. They argued that gas warfare would divert aircraft from the more effective strategy of bombing Germany’s industries and cities. Britain’s gas attacks would not be decisive, they feared, and Germany would probably retaliate with devastating effect against England and might also use gas elsewhere in Europe and possibly against Allied prisoners of war.

Churchill complained to an associate that he was “not at all convinced by this negative report,” but he reluctantly yielded. “Clearly I cannot make head against the parsons and the warriors at the same time,” he lamented privately.

His advisers had also considered bacteriological warfare —probably anthrax, code-named “N.” It “is the only Allied biological agent,” the Joint Planning Staff, advisers to the British military chiefs, reported, “which could probably make a material change in the war situation before the end of 1945. There are indications which lack final scientific proof, that the 4-lb. bomb charged with 'N,’ used on a large scale from aircraft might have a major effect on the course of the war.” The Joint Planning Staff concluded that Britain, dependent on the United States for “N,” would still lack adequate stocks of it well into 1945. Had supplies been ample, however, Churchill might have faced a tempting military prospect.

In the United States, a relatively powerless group, seeking to halt Hitler’s relentless gassing of Europe’s Jews as part of the “final solution,” urged Roosevelt to threaten Hitler with gas warfare if Germany did not stop its program. These petitions predictably failed. The Joint Chiefs, to whom the pleas were sent, concluded the matter was not in “their cognizance.” And Hitler never used gas against Allied armies, probably because he feared retaliation and recalled his own gassing of 1918.

Despite Roosevelt’s pledge against gas, the United States Army hoped in 1945 to initiate gas warfare against Japan. On several occasions Gen. George C. Marshall, Army chief of staff, wanted to use it in the Pacific. The first time, after the heavy casualties at Iwo Jima in February and March, Marshall proposed using the weapon on Okinawa before the invasion, then deemed likely to cost thousands of American casualties. Gas warfare, as Marshall later explained, would have pushed the inhabitants to a remote part of the island and kept the Japanese troops in gas masks for about a week, thus so weakening them that the invasion “could have been accomplished with little loss of life.” In recalling these plans, Marshall never mentioned that gas was inhumane. His implication seemed clear: The efforts to save American lives overrode the constraints of morality.

Why, then, wasn’t the gas used? Marshall later claimed that the chief reason was the opposition of the British, who feared that Germany, caught in the last weeks of war, might use the weapon in Europe. Marshall implied that Roosevelt might have repudiated his pledge and sanctioned America’s initiation of gas warfare. There are no records of any conversation with Roosevelt on this matter, however, and probably Britain’s fears sufficed to deter Marshall from raising the issue with FDR in the early spring of 1945.

With the defeat of Germany on May 8, such fear of retaliation in Europe evaporated. Accordingly Gen. Joseph Stilwell, the former commanding general of Army ground forces in China, recommended, only a few weeks after President Roosevelt’s death, that gas be used in the invasion of Japan. Disregarding Roosevelt’s repeated and adamant public statements, Stilwell said, “We are not bound in any way not to use it, and the stigma of using it on the civilian population can be avoided by restricting it to attack on military targets.”

At a special session on May 29 with Secretary of War Stimson, according to a recently declassified document, General Marshall pushed for gas “to cope with the … last ditch defense tactics of the suicidal Japanese.” Appalled by American casualties in the battles on the outlying islands, Marshall argued the case for gas warfare: “It did not need to be our newest and most potent—just drench them and sicken them so that the fight would be taken out of them—saturate an area, possibly with mustard. …”

He admitted that public opinion might be a problem but concluded that it could be dealt with. After all, he argued, gas was “no less inhumane than phosphorus and flame throwers and need not be used against dense populations or civilians—merely against these last pockets of resistance which had to be wiped out but had no other military significance.”

Had the Pacific war dragged on, Truman might have been pressed to use gas against the Japanese.

The issue did not dominate this May 29 meeting, however, for Stimson and Marshall were primarily concerned with the use of the atomic bomb. Marshall, while willing to violate the moral code against gas warfare, was reluctant to use the bomb against civilians. He recommended that it “might first be used against straight military objectives such as a large naval installation and then if no complete result was derived from the effect of that… we ought to designate a number of large manufacturing areas from which the people would be warned to leave—telling the Japanese that we intended to destroy such centers.”

Just two days later, however, the Interim Committee, a high-level advisory group on the A-bomb, “agreed that the most desirable target would be a vital war plant employing a large number of workers and closely surrounded by workers’ houses.” This would be, in effect, terror bombing—with mass deaths designed to frighten the living into surrendering before they suffered a similar fate.

Untroubled by his defeat on the A-bomb, Marshall continued to argue against FDR’s pledge not to use gas. He soon found an ally in Gen. Douglas MacArthur, commander in the Pacific, who could see “no reason why we should not use gas right now against Japan proper. Any kind of gas.” Unlike Marshall, MacArthur was not hesitant about killing civilians or using the most poisonous gases.

Marshall also received important support for gas warfare from Assistant Secretary of War John J. McCloy, who argued for reconsidering policy “in the face of the public pressure for the use of gas, which may develop as our casualties rise due to the Okinawa cave type of Japanese defense.” McCloy, a distinguished Wall Street attorney, seemed perfectly comfortable about redefining the ethics of war. In the crucible of World War II, morality had been substantially altered; saving American lives and insulating the military from public criticism were McCloy’s main concerns.

At Marshall’s behest the Army’s Operations Division (OPD) in early June put together a paper offering both new and familiar rationales for using gas in the Pacific: It would save American lives, and the British no longer feared German retaliation. There was, however, a serious danger that Japan would retaliate against noncombatant populations, especially in China and in Manchuria and Korea, although such retaliation would be “only to a limited extent.” And the OPD acknowledged that the introduction of gas would erode moral restraints but concluded that this made no practical difference, since chemical and biological warfare in any future conflict would be directed against the United States “on the opening day.”

American public opinion, the OPD report optimistically concluded, easily could be shifted to accept gas warfare. “A program of education, stressing [that it is not worse than flamethrowers, phosphorus, or napalm] and that lives of … soldiers can be saved, will overcome this prejudice. Actually, there is considerable public demand to use gas,” the OPD emphasized. Support for gas warfare, near 40 percent according to public opinion polls, had been growing in the months since Iwo Jima.

By mid-1945, as Army planners knew, Japan had produced very little gas and, lacking air superiority, could not use it against American troops outside the main Japanese islands. While the United States had produced about 135,000 tons of chemical warfare agents, Germany about 70,000 tons, and Britain about 40,000 tons, Japan had only 7,500 tons. In brief, American production was 1,800 percent greater than Japan’s.

But if the Army viewed gas warfare as useful in softening Japan, the invasion plans did not hinge upon approval of gas warfare, and planners saw that the use of such a weapon could depend upon Allied agreement. Accordingly the OPD suggested that President Harry S. Truman discuss the issue with Joseph Stalin at Potsdam and then with Chiang Kai-shek.

In Washington, General Marshall sent the OPD report to the other military chiefs. There is no record of replies by Admiral King or by Gen. Henry H. (“Hap”) Arnold, chief of staff of the Army Air Forces. The Navy, in view of its faith in bombing and a blockade, had its own agenda for ending the war and was probably not supportive of Marshall’s plan.

Arnold had earlier rejected a gassing plan—”a quick knockout of Japan from the air by concentrating on sources of food,” partly by spraying mustard gas on rice-producing areas—on tactical rather than on moral or political grounds. As one of his aides had explained, “the effort to do a good job against food would be better expended against material objectives having earlier and certain impact.” Given limited resources, the Air Force preferred to continue its bombing of Japanese cities, which some Air Force generals thought might defeat Japan before the planned invasion in November.

Alone among the President’s top military advisers, Adm. William Leahy, the crusty, aged chief of staff to the Commander in Chief, opposed Marshall’s plan. And unlike many of the top brass, Leahy was not reluctant to raise fierce moral objections. Earlier he had opposed both gas and bacteriological warfare because, as he had told FDR in 1944, they “would violate every Christian ethic I have ever heard of and all of the known laws of war. It would be an attack on the noncombatant population of the enemy.” On June 20, 1945, replying sharply to Marshall, Leahy emphasized that Roosevelt had categorically barred first use of gas.

Apparently Marshall never brought his plans to Truman. The last important reference on the matter appears in an OPD briefing paper for the Potsdam Conference: “the advisability of changing the policy to permit the use of gas against the Japanese has been discussed informally by the Joint Chiefs of Staff. [Because the Allies might oppose such a reversal in policy], a decision to initiate use of gas must be taken on the highest level.”

If he had wanted to, could Truman have reversed Roosevelt’s public commitment? What tactics might he have successfully employed? He could have lied and claimed publicly that Japan had recently initiated gas warfare and that the United States was only retaliating. But such deceit could have backfired and would have been politically risky. As a top-secret Army report warned, “the probability that our decision to adopt gas warfare could be long hidden under the cloak of a framed incident is small.”


And despite the optimism of Army planners about public opinion, an open presidential admission of a policy reversal would also have been politically dangerous. The American people, though inured to the intentional killing of civilians by bombing, might still have protested against gas. For years it had been condemned as immoral, and throughout the war most nations—including Germany—had seemed to abide by that ethical code in combat.

By mid-1945 the injury to American prestige and power would not have been worth the military advantages of violating the accepted morality. Perhaps if a handful of respected advisers all had argued the necessity of a reversal, Truman might have changed policy. But not even Marshall argued for the necessity of gas warfare; he only said it would be useful. Finally, it would have been difficult for Truman to justify a rejection of FDR’s public pledge. Whereas Roosevelt was an architect of the use of the atomic bomb, he was a powerful opponent of gas warfare. In each case FDR’s legacy, carried in part by the advisers he had bequeathed to his successor, narrowed the range in which the new President could make decisions. And in each case that legacy probably also fitted Truman’s own inclinations.

Yet any analysis of this question of what might have been is, to use FDR’s word, iffy. Had the Pacific war dragged on into the late autumn and winter, Truman might have been under growing pressure to use gas against the hated Japanese. The costly struggle was eroding American repugnance to gas, and future battles in Japan, with thousands of GI deaths, might well have led American citizens to push their government to use gas warfare. Under those pressures only a secure and powerful President like FDR, with a firm commitment against gas, might have chosen to resist. Truman, less secure and not wedded to that commitment, might have yielded more easily, especially after the atomic bombings. Truman later wrote, “The Atomic bomb … is far worse than gas or biological warfare because it affects the civilian population and murders them wholesale.”

And in his last years Truman kept on his bookshelf, next to volumes about the A-bomb decision, a copy of Hamlet, with Horatio’s speech in the last act underlined:

let me speak to the yet unknowing world How these things came about: so shall you hear Of carnal, bloody, and unnatural acts, Of accidental judgements, casual slaughters, Of deaths put on by cunning and forced cause, And, in this upshot, purposes mistook Fall’n on the inventor’s heads. … But let this same be presently perform’d, Even while men’s minds are wild; lest more mischance, On plots and errors, happen.

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