I believe it was between 1939-1942. This claim was also paired with they found Chinese and Japanese officers who were working together to bloodlessly take and retake cities to look good for their commanding officers. The book is the William Donovan Biography by Richard Dunlop.

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It also has the claim in it that Szmul Zygielbojm specifically killed himself because FDR refused to bomb Auschwitz after Szmul gave proof to Arthur Goldberg, who gave it to William Donovan, who showed it to FDR, FDR refused to redirect planes, this gets back to Szmul, and he kills himself in protest. No mention of any of this or even a connection between the people involved on Wikipedia.

Has anyone else ever heard the claim that Chang Kai-shek sold American weapons to some Japanese? This is reported in a William Donovan biography as one of the first few things discovered when the OSS sent someone to China.