“In the autumn of 1936, a message from a high military source in Germany was conveyed to President Beneš [President of Czechoslovakia – E.S.] to the effect that if he wanted to take advantage of the Fuehrer’s offer, he had better be quick, because events would shortly take place in Russia rendering any help he could give to Germany insignificant.
While Beneš was pondering over this disturbing hint he became aware that communications were passing through the Soviet Embassy in Prague between important personages in Russia and the German Government. This was a part of the so-called military and Old-Guard Communist conspiracy to overthrow Stalin and introduce a new régime based on a pro-German policy. President Beneš lost no time in communicating all he could find out to Stalin. Thereafter there followed the merciless, but perhaps not needless, military and political purge in Soviet Russia, and the series of trials in January 1937, in which Vyshinsky, the Public Prosecutor, played so masterful a part.
The Russian Army was purged of its pro-German elements […] The bias of the Soviet Government was turned in a marked manner against Germany. […] The situation was, of course, thoroughly understood by Hitler; but I am not aware that the British and French Governments were equally enlightened. To Mr. Chamberlain and the British and French General Staffs the purge of 1937 presented itself mainly as a tearing to pieces internally of the Russian Army, and a picture of the Soviet Union as riven asunder by ferocious hatreds and vengeance.”
– Winston Churchill, “The Gathering Storm: The Second World War, Volume 1,” page 258.