FDR's second ambassador to Moscow, Joseph E. Davies (1876 - 1958), wrote this stunning article in which he makes clear that he was all in favor of Stalin's purges and believed that the trials "indicated the amazing far-sightedness of Stalin and his close associates". He believed every one of the trumped-up charges and swallowed them hook, line and sinker. He concluded the article by advising other "liberty loving nations" to follow Stalin's example.
Davies wrote a memoir encapsulating all his adoration for Stalin and titled it Mission to Moscow (1941). Warner Brothers made a highly unsuccessful movie of the book, releasing it under the same name title, you can read about it here.
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