Paul Rogers, 4. august 2005

The dropping of the atomic bombs on Hiroshima and Nagasaki in August 1945 remains a focus of historical controversy even sixty years later. Until the 1970s, most historians accepted the view that the weapons were used to prevent the need for an American invasion of the Japanese mainland scheduled to start in November 1945. Subsequently, “revisionist” critics argued that there were other motives, not least the need to bring the war to an early end because of the rise of Soviet influence in the region. In this view, Hiroshima and Nagasaki were early shots in the cold war, at a time when Japan was actually close to surrender.

A later generation of historians argued that the release of previously secret intelligence material dating from early 1945 establishes that Japan was not ready to surrender, and that its army planned to defend the homeland so formidably, and at such cost to American invaders, that Japan could force a ceasefire on favourable terms (see Richard B Frank, "Why Truman Dropped the Bomb", Weekly Standard, 8 August 2005).

The arguments and counter-arguments will not easily be concluded, and it is certainly the case that the United States was prepared to continue the use of atomic weapons against Japanese cities until surrender was forced; the Manhattan Project was thought to be capable of producing two more atom bombs a month through to the end of 1945. In the event, the formal Japanese surrender on 2 September 1945 brought the war to an end without further nuclear attacks.

But what would have happened if the Manhattan Project had not had its effect and the United States’s projected invasion of Japan had indeed gone ahead? The sudden end of the war precipitated by the two atomic bombs, and subsequent secrecy on the part of the United States, disguised for many years the fact that the US had prepared a remarkable back-up plan. This was the mass-production of enormous quantities of chemical weapons to be used against Japanese cities, that envisaged killing as many as 5 million people.

This previously secret plan came to light with the declassification of sensitive papers after the end of the cold war, and was written up some years later in a paper for the authoritative Proceedings of the US Naval Institute by two military historians, Norman Polmar & Thomas B Allen ("The Most Deadly Plan", Proceedings, January 1998).

It scarcely reached the public domain at the time, yet it says much about the approach to warfare that had developed by 1945, including a willingness to inflict mass civilian casualties on a scale far higher even than the carpet-bombing of Tokyo, Hamburg or Dresden or the atom bombs on Hiroshima and Nagasaki.

As the war in Europe was being fought to a bitter end during the winter of 1944-45, there was still uncertainty that the Manhattan Project would succeed in its aim of producing atomic weapons. By early 1945, plans were already underway for an American invasion of the Japanese mainland, “Operation Olympic” (the first part of the wider “Operation Downfall”), which was to start with the southern island of Kyushu.

The high American death toll in the much smaller invasions of the Marianas, Iwo Jima, Okinawa and elsewhere led the US army's Chemical Warfare Service to devise a plan to use massive chemical weapon attacks to support the invasion of the mainland.

The details were contained in A Study of the Possible Use of Toxic Gas in Operation Olympic, an anodyne title for an extraordinary proposal that involved two different if almost simultaneous uses for thousands of tons of chemical weapons. The main weapons to be used were two chemical blister agents, phosgene and mustard gas, together with hydrogen cyanide and cyanogen chloride.

At the time of the invasion itself, tactical strike aircraft would drop nearly 9,000 tons of chemical weapons on the defending troops in the first fifteen days, with further attacks planned at the rate of just under 5,000 tons every thirty days from then on. As US troops came ashore, they would bring in howitzers and mortars that could deliver an additional forty-five tons a day of poisonous gas on Japanese positions.

This represented a massive use of chemical weapons, but it was dwarfed in scale by the proposed attacks on Japanese cities. In what the document described as an "initial gas blitz", long-range B-29 and B-24 strategic bombers would attack a large number of cities across Japan – starting with Tokyo, fifteen days before the ground invasion started. Over the next, initial fifteen-day period, over 56,000 tons of gas bombs would be dropped on cities, followed by almost 24,000 tons of gas bombs dropped every month from then on until the war ended or all the planned targets had been hit.

Although this plan was completed only in June 1945, it originated in work started by the Chemical Warfare Service more than eighteen months earlier; as early as April 1944, a detailed study – Selected Aerial Objectives for Retaliatory Gas Attacks on Japan – had been completed assessing the vulnerability of cities such as Tokyo, Nagoya and Osaka to gas attack. The analysts believed that their densely populated residential areas, with narrow streets and few open spaces, were particularly susceptible to chemical warfare. Moreover, mustard gas is readily absorbed by wood, and Japanese wooden houses would have been very difficult to decontaminate.

The intention was to maximise casualties, mostly civilian, and the study stated:

By June 1945, the full gas-attack plan was submitted to Major General William N Porter, head of the Chemical Warfare Service, detailing fifty urban and industrial targets, including twenty-five cities that were particularly susceptible to gas attack. According to the report, "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 chemical warfare attacks were never implemented, but the programme was in no sense theoretical. While the plans were being formulated, much effort was put into manufacturing and stockpiling the weapons so that they would be ready if needed. The first chemical weapon plant had been opened in April 1944 at Warners, New York state, initially producing about eighty tons of poison gas a week. This was later increased to over 400 tons a week, and more plants were built so that by 1945 the US army had over fifty million chemical artillery shells and the US army air corps had more than a million bombs and 100,000 aircraft spray tanks.

The dropping of the atom bombs on Hiroshima and Nagasaki remains controversial to this day. The fact that many more atom bombs would have been used against Japanese cities if the imperial government had not surrendered when it did indicates the determination of the US leadership to end the war with the lowest possible American casualties, whatever happened in Japan.

If the Manhattan Project had not succeeded in producing "Little Boy" and "Fat Man", then "Plan B" was waiting in the wings, potentially capable of killing even more people than died in Hiroshima, Nagasaki, or the firebombing of Tokyo.