Having deserted out of revulsion of his troops' "true, inhuman mission" in 1944-the only one to do so among the Japanese BW troops' 12,000 personnel, Chimba spoke about "a great scourge" inflicted on the Chinese provinces of Zhejiang and Jiangxi in May 1942 by Ishii and his scientists, whose powerful weapons, apart from plague, cholera, typhoid and dysentery bacteria, included anthrax.

The confessions of Chimba met with a similar fate as all other incriminating evidence-Sutton focused his attention on prosecuting the Japanese for the atrocities of the Nanking Massacre and "almost nothing concerning BW surfaced at the Far East International Military Tribunal's war crimes trials", Harris said.

However, in September 1946 the highly incriminating revelations of Major Karasawa and three other Soviet captives were brought to the attention of the Americans by those from the Soviet Division of the IPS, who demanded to interview Ishii for a potential war crimes trial.

If anything, this only served to convince the Americans of the value of their prize find and the need to keep him for themselves, away from the Russians with whom the decisionmakers in Washington believed they had entered a competition to develop the most powerful biological weapons.

"Up to that point, Ishii had consistently lied about the issue of human experiments, largely denying the fact that they had ever been conducted," Wang said. "Armed with the affidavits provided by the Soviets, the Americans, believing that they could exploit Ishii's fear of the Soviets to their own advantage, confronted the sly general, who then seized this opportunity to put forward his demand for the bargain-immunity from 'war crimes' in documentary form for himself, superiors and subordinates."

On May 6, 1947 MacArthur sent a top-secret radiogram to the War Department's General Intelligence Division, requesting action by the Joint Chiefs of Staff on an immunity agreement for the Unit 731 scientists.

In it, MacArthur said the affidavits from Karasawa and his superior Major General Kyoshi Kawashima had been "confirmed tacitly by Ishii", who "claims to have extensive theoretical high-level knowledge including strategic and tactical use of BW on defense and offense, backed by some research on best BW agents to employ by geographical areas of the Far East and the use of BW in cold climates".

The final deliberation from Washington is laid out bluntly in the Aug 1, 1947 intelligence report for circulation inside the US State-War-Navy Coordinating Sub-Committee for the Far East.

"This Japanese information is the only known source of data from scientifically controlled experiments showing the direct effect of BW agents on man... any war crimes trial would completely reveal such data to all nations, it is felt that such publicity must be avoided in the interests of defense and national security," the report said, noting at the same time that:"Experiments on human beings similar to those conducted by the Ishii BW group have been condemned as war crimes by the International Military Tribunal for the trial of major Nazi war criminals."