The Most Deadly Plan

By Norman Polmar and Thomas B. Allen

When landings began in November, tactical fighters and attack planes were to drop another 8,971 tons in the first 15 days, followed by 4,984 tons of bombs every 30 days. Other planes would swoop low, using spray tanks to spread thousands of tons of liquid gas over Japanese defenders. During the landings, U.S. troops would bring ashore 67 Army battalions of 105-mm and 155-mm howitzers and 4.2-inch mortars that were to fire about 1,400 tons of gas shells every 30 days.

Against unprotected troops—or civilians—these weapons would have been devastating. Against protected troops they would have caused casualties and, equally important, forced defenders to fight in restrictive gas masks and protective capes. Thus far in the Pacific War, neither side had used chemical weapons.

This proposal was the culmination of more than a year and a half of intensive planning by the Army's Chemical Warfare Service. Senior military leaders first considered the concept in November 1943, after almost 1,000 U.S. Marines were killed taking Tarawa Atoll from determined and fanatical defenders . A month later, General Porter wrote: "The initiative in gas warfare is of the greatest importance. We have an overwhelming advantage [over the Japanese] in the use of gas. Properly used gas could shorten the war in the Pacific and prevent loss of many American lives." 1 The core rationale of U.S. military commanders was simple; they perceived the use of chemical—and also biological—weapons as a way to save lives. And the option remained on their minds as Allied assaults moved closer and closer to the Japanese home islands.

In April 1944, U.S. Army chemical warfare and air intelligence officers prepared "Selected Aerial Objectives for Retaliatory Gas Attack on Japan," highly detailed studies of Tokyo, Nagoya, Osaka, Yawata, and adjacent cities and suburban areas. This analysis examined the climatic conditions, construction characteristics, street layout, and other features of Japanese cities to determine vulnerability to gas attacks.

The secret 1944 report noted, for example, "The gas attack program is aimed primarily at causing the maximum number of casualties, crippling transportation and public services, complicating and delaying the repair of HE [high explosive] bomb damage and making targets more vulnerable to incendiary attack."

Japanese cities were particularly vulnerable to gas attack, notably because residential areas were constructed almost entirely of wood, and "Liquid mustard [gas] is readily absorbed by wood which is almost impractical to decontaminate."

Meanwhile, U.S. forces were pushing relentlessly toward the Japanese home islands, using conventional weapons and methods for amphibious landings on Peleliu, the Marianas, the Philippines, Iwo Jima, and, on 1 April 1945, Okinawa. The landings were becoming more costly, however; tenacious Japanese defenders ashore were being aided by kamikaze attacks against U.S. warships and transports.

The U.S. death toll was inciting strong reactions. "You can cook them with Gas," read an editorial headline in The Chicago Tribune on 11 March 1945, as U.S. troops were fighting on Luzon and Iwo Jima with heavy losses. The editorial declared the charge that poison gas "is inhumane" as "both false and irrelevant. ... The use of gas might save the lives of many hundreds of Americans and of some of the Japanese as well."

In May 1945, General Joseph Stilwell, soon to take command of the Tenth Army on Okinawa, wrote to General of the Anny George C. Marshall, Anny Chief of Staff, about the pending invasion of Japan. His suggestions concluded: "Consideration should be given to the use of gas. 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."

On 29 May 1945, General Marshall told Secretary of War Henry Stimson:

... of gas and the possibility of using it in a limited degree, say on the outlying islands where operations were now going on or were about to take place. [Marshall] spoke of the type of gas that might be employed. 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, and just stand off. ... The character of the weapon was no less humane than phosphorous [sic] and flame throwers and need not be used against dense populations or civilians-merely against those last pockets of resistance that had to be wiped out but had no other military significance." 2

On 9 June 1945, three officers of the Army's Chemical Warfare Service submitted the ultimate, top-secret gas attack plan to General Porter, who approved it.

The Army planners had chosen 50 "profitable urban and industrial targets," with 25 cities listed as "especially suitable for gas attacks." The report declared, "Gas attacks of the size and intensity recommended on these 250 square miles of urban population ... might easily kill 5,000,000 people and injure that many more."

The use of gas was to begin 15 days before the landings—starting with a drenching of much of Tokyo, because an "attack of this size against an urban city of large population should be used to initiate gas warfare." Planners targeted 17.5 square miles directly north of the Imperial Palace and west of the Sumida River. Almost a million people would be in that area at the time of the first strike. Within two miles of the target area were 776,000 more Japanese; they probably would be in the path of wind-carried gas. (Ironically, the size of the targeted area was almost exactly the same as the area of Tokyo burned out by the B-29 firebombing on the night of 9-10 March 1945. But the chemical warfare planners made no reference to bombing damage to cities on the target list.)

The attack on Tokyo was to begin at 0800, when the greatest number of people would be concentrated in the city. Four-engine B-29 and B-24 bombers would drop either 21 ,680 gas bombs weighing 500 pounds or 5,420 bombs weighing 1,000 pounds, depending upon their availability. All would be filled with a gas known as phosgene.

During subsequent attacks on other Japanese targets—both by U.S. aircraft and artillery—three additional types of gas—hydrogen cyanide, cyanogen chloride, and mustard—were set to be used. (Phosgene and mustard gas had caused many thousands of casualties in World War I.)

Mustard gas would be used against Yawata and the nearby cities of Tobata, Wakamatsu, and Kokura, a highly industrialized area. The objective was to "hamper operations and produce mustard gas vapor casualties" among the 279,200 people in the gas attack zones. "Refresher attacks" would launch every six days until the first frost.

In direct support of the invasion of Kyushu, cyanogen chloride bombs were to be dropped on Japanese troop units around Goshima, the chief city on Kyushu. Raids on reserve troops, however, would likely "produce large numbers of casualties among the unprotected urban population of Kagoshima." Gas attacks, the report continued, "should be coordinated with the 'softening up' bombardment of the beaches prior to landing."

Extremely detailed analyses of city layouts even paid attention to the width of streets and the location of parks. Discussing one gas-attack zone in Yokohama, for example, the report said: "Zone I covers the center of the city proper, a triangular area congested with residential and mercantile structures. This is the most densely populated region in the city. Dense clusters of low residences, broken only by narrow streets, extend inward. The northern and western parts of this district are covered with cheap native shops and theaters. There are no large factories in this zone and comparatively few household shops."

Army planners believed that Japanese officials would not evacuate cities, even after the first wave of poison-gas attacks, because of the strain that mass evacuation would place on the transportation system and because workers were needed to keep factories operating.

Target selection was based on the thesis that "... most Japanese cities of over 100,000 population are located on or very near the coast, a fact of significance for gas attack because it aids identification and exposes them to daily land-sea winds .... There are few open spaces in most Japanese cities. There are a number of parks in Tokyo but few elsewhere ...." Noting that about 70% to 80% of the roofs in typical cities were tiled and the rest sheet metal, the report says that both types "are easily penetrated by gas bombs."

Cities were "studied in considerable detail for the purpose of preparing gas zone maps," depending upon the density of population. The greater the concentration of people, the better the gas target.

Only five copies of the top-secret report of 9 June were made. On 14 June, other documents show, Fleet Admiral Ernest J. King, Commander in Chief, U.S. Fleet, received a secret report on the use of poison gas from General Marshall.

The timing of the discussion between King and Marshall strongly suggests that poison gas was discussed at the 18 June 1945 meeting in the White House on the invasion of Japan. The minutes of that meeting refer to other, undisclosed topics. One was revealed later to be the atomic bomb.

The newly available report of 9 June strongly indicates that another object of discussion at the White House on 14 June was the massive use of poison gas.

On 21 June, according to one of the documents, orders went out to step up production of several poison-gas types so that stockpiles could satisfy the massive amounts urged in the plan.

The Army's Edgewood (Maryland) Arsenal and civilian laboratories already were researching toxic gases extensively, and factories were producing toxic materials as well as weapons. The first poison-gas plant was at Warners in New York state, which opened in April 1944. Initially, it produced 15 tons of cyanogen chloride per day, later increasing to 60 tons per day. By mid-1945 several additional plants were producing toxic gases, and several more remote areas were being used to test them. In 1942 a chemical test area had been set up in the desert wasteland of Utah, including part of the Dugway Valley, and at Fort Sibert in Alabama. Other chemical test sites, with more tropical conditions, were at Bushnell, Florida, and San Jose Island in Panama. Toxic U.S. weapons were also tested on Brook Island in the Australian state of Queensland, as well as at smaller test facilities in Canada and India. (Massive numbers of U.S. toxic weapons were also stored in Australia.)

Weapons intended to destroy tanks, planes, or even buildings could be tested against inanimate objects. But toxic gases were meant for human targets. Thus, volunteers—thousands of U.S. and Australian servicemen—were experimentally exposed. The precise number of men on whom gases were tested may never be known, but the U.S. Navy alone had at least 65,000 test volunteers from the Great Lakes training station near Chicago. 3

The Army and the Navy conducted several research projects, and the Navy had a 1O-by-15-by-17-foot chemical weapon test chamber at the Naval Research Laboratory in Washington, D.C., across the Potomac River from the suburb of Alexandria, a few miles from the Capitol. Several men could be exposed to gases for up to an hour to test the effectiveness of gas as well as protective clothing, masks, and ointments. The volunteers were alternately exposed to the gas wearing ordinary and protective clothing, plus gas masks. Although the volunteers were monitored closely and examined before and after the tests, many suffered injuries. 4 In Australia, volunteers wearing normal clothing and gas masks worked moving sandbags in the gas chambers. Many were hospitalized, often with the skin around the scrotum and underarm burned raw. 5

By 1945, almost 51 million chemical artillery shells, more than 1,000,000 chemical bombs, and more than 100,000 aircraft spray tanks were available. 6 Also in the new arsenal were mustard-gas land mines made from rectangular, one-gallon tin cans, commonly used for varnish or syrup. The ten pounds of mustard in the can were detonated by a slow-burning fuse or electrical current, and the gas would spread over a considerable area. They were intended as booby traps or for contaminating fields, roads, or buildings. By April 1945 more than 43,000 such mines reposed in Pacific stockpiles. 7 Other chemicals were mass-produced but were not used in specific weapons.

This huge U.S. chemical arsenal was intended for a multi-front war, supporting U.S. combat operations in Europe and the Mediterranean as well as the Pacific and Southwest Pacific areas. In 1945, it was proposed, all of this arsenal of toxic gases would be used to drench the Japanese home islands—creating some 5,000,000 Japanese casualties while saving countless U.S. lives.

The exchange between General Marshall and Admiral King on 14 June 1945 addressed the fanatical Japanese resistance being encountered in the Pacific. Marshall's memorandum to King stated: "Gas is the one single weapon hitherto unused which we can have readily available which assuredly can greatly decrease the cost in American lives and should materially shorten the war." 8

Only 50-odd years later has the scope of the plan for that weapon's ultimate use come to light.

1 Memorandum from MGEN William N. Porter to LGEN Joseph McNamey, 17 Dec 1943; War Department file OPD 385.

2 Assistant Secretary of War John J. McCloy, Memorandum of Conversation with GEN Marshall, "Objectives towards Japan and Methods of Concluding the War with Minimum Casualties," 29 May 1945.

3 Rexmond C. Cochrane, "Medical Research in Chemical Warfare" (Aberdeen, MD: Aberdeen Proving Ground, U.S. Army [n .d.; 1946]), Monograph, 163, 167.

4 See, for example, Naval Research Laboratory, "Chamber Tests with Human Subjects, IX. Basic Tests with H Vapor," in Constance M. Pechura and David P. Rail (ed.), Veterans at Risk: The Effects of Mustard Gas and Lewisite (Washington, D.C.: National Academy Press, 1993), p. 361.

5 Karen Freeman, "The Unfought Chemical War," The Bulletin of Atomic Scientists (December 199 1), p. 32.

6 A detailed breakdown of chemical munitions is provided in Leo P. Brophy, et. al., The Chemical Warfare Service: From Laboratory to Field (Washington, D.C.: Center of Military History, U.S. Army, 1988), p. 65.

7 Ben R. Baldwin, et. al., Readiness for Gas Warfare in the Theaters of Operations (manuscript in National Archives), pp. 449, 454.

8 Memorandum from GEN Marshall to ADM King, 14 June 1945, enclosing "Memorandum entitled U.S. Chemical Warfare Policy."

Mr. Polmar and Mr. Allen are coauthors of Codename Downfall: The Secret Plan to Invade Japan and Why Truman Used the Atomic Bomb (New York: Simon & Schuster, 1995). Their Proceedings article "Invasion Most Costly" (August 1995) won them the Naval Historical Center's Rear Admiral Ernest M. Eller Award.


Thomas B. Allen is well known for his writing in the subject of espionage and military history. His books include Declassified: 50 Top-Secret Documents That Changed Military History. He is also the co-author with Norman Polmar of Spy Book: The Encyclopedia of Espionage, and an encyclopedia of World War II: America at War 1941-1945. His writings on intelligence and related topics have appeared in such national publications as Smithsonian and The Washington Post Magazine, in addition to the Naval Institute's Proceedings and Naval History magazines.

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