The USSR secretly helped the German army rebuild and militarize, directly in violation of

So wrong that it hurts. Actually, Germany and the Soviet Union were both pariah states after WW1 and cooperation had already begun with the Treaty of Rapallo in 1922, eleven years before Hitler become chancellor of Germany. The so called Black Reichswehr trained within the USSR as early as 1922, way before the NSdAP gained traction in German elections. Later, due to the Molotov-Ribbentrop Pact which entailed the partition of central and eastern Europe between Germany and the USSR in 1939 nobody in the Soviet Union was surprised.