The Holocaust Historiography Project

American Leaders Planned Poison Gas Attack Against Japan

by Mark Weber

A long-suppressed report written in June 1945 by the US Army’s Chemical Warfare Service shows that American military leaders made plans for a massive preemptive poison gas attack to accompany an invasion of Japan. The 30-page document designated “gas attack zones” on detailed maps of Tokyo and other major Japanese cities. Army planners selected 50 urban and industrial targets in Japan, with 25 cities, including Tokyo, Osaka, Yokohama, Kobe and Kyoto, listed as “especially suitable for gas attacks.”

In planning the invasion of Japan proper, America’s military and political leaders expected the Japanese to fight with fanatic fervor in defense of their home islands. The overall US plan, code-named “Operation Downfall,” called for a two-stage invasion. An assault on the southernmost Japanese home island of Kyushu, code-named “Operation Olympic,” was set for November 1, 1945. This was to be followed by “Operation Coronet,” scheduled for March 1946: an invasion of the main Japanese home island of Honshu, including an assault on Tokyo.

“Gas attacks of the size and intensity recommended on these 250 square miles of urban population,” the US Army report declared, “might easily kill 5,000,000 people and injure that many more.” In the first attack, which would be launched 15 days before the Kyushu landings, American bombers would drench much of Tokyo and other cities in an early morning attack with 54,000 tons of lethal phosgene gas. Tokyo would be the largest poison gas target, because an “attack of this size against an urban city of large population should be used to initiate gas warfare.”

The report’s three authors recommended that the US Joint Chiefs of Staff issue “a policy at once directing the use of toxic gas on both strategic and tactical targets in support of Operation Olympic.” Planners called for the use of four kinds of gas, including phosgene (or carbonyl chloride), mustard gas, and hydrogen cyanide. The gas attack study was approved by the chief of the US Chemical Warfare Service, Major General William N. Porter. Only five copies were made of the top secret document, whose existence was first made public in July 1991.

After the horrific use of poison gas during the First World War, the major nations formally outlawed the use of this new weapon. This prohibition was included in the 1919 Treaty of Versailles, the 1922 Treaty of Washington, and in a 1925 protocol signed by more than 40 countries, including the United States. During the Second World War, both the United States and Germany produced and stockpiled lethal gas for possible use in the European conflict, but neither side — apparently fearful of retaliation — actually used the weapon.

Although the public policy in 1945 was that the United States would use gas only in retaliation for a Japanese first use, in private America’s military leaders seriously considered striking first with poison gas. By the summer of 1945, American forces were already killing Japanese by the tens of thousands in indiscriminate fire-bombings. Given this, the step to killing by lethal gas was not a lengthy one.

On June 14, 1945, other documents show, Fleet Admiral Ernest King, a member of the Joint Chiefs of Staff, received a secret report on poison gas from Army Chief of Staff General George C. Marshall. The two men were key presidential advisers. President Truman met at the White House on June 18 with his principal military and civilian advisers to discuss the overall plan for the invasion of Japan. Apparently the gas attack plan was approved at that conference. Three days later, June 21, orders were given to step up production of several types of poison gas to provide stockpiles in the massive quantities urged in the study.

Two American historians, Thomas B. Allen and Norman Polmar, commented on the long-suppressed document in a 1995 article. The June 1945 report, they wrote, “raised the killing of enemy civilians to a level far beyond anything seen in World War II. No [other] known military document from World War II recommends such wholesale killing of civilians.” (T. B. Allen and N. Polmar, “Poisonous invasion prelude,” Pittsburgh Post-Gazette, Aug. 4, 1995 [New York Times special features].)

No American official has ever been demoted or even criticized for approving this murderous plan, which has received scant public attention. If Germany had used poison gas during the Second World War, surely the victorious Allies would have severely punished the responsible officials. Similarly, if German military leaders had approved a plan to gas London comparable to the 1945 American one to drench Tokyo in phosgene, doubtless it would have been cited endlessly as a striking example of Nazi evil, and those responsible for drafting it would have been vilified.