Before the German-Soviet pact of Ribbentrop and Molotov, the roots of cooperation between the two totalitarianisms

https://en.wikipedia.org/wiki/Lipetsk_fighter-pilot_school

https://en.wikipedia.org/wiki/Treaty_of_Rapallo_(1922)

One must read this paper:
https://www.jstor.org/stable/1986524?seq=1#page_scan_tab_contents

also:

http://avalon.law.yale.edu/20th_century/rapallo_001.asp

https://books.google.fr/books?id=s42U33iW_nYC&pg=PT306&lpg=PT306&dq=Dyck,+Harvey+Leonard.+Weimar+Germany+%26+Soviet+Russia+1926-1933+(1966)&source=bl&ots=EisHTWpPS8&sig=0isNt7v2IKAZ7qIIQ6d6t4ev0bk&hl=en&sa=X&ved=0ahUKEwiDiqiXm8XXAhXFIlAKHa2oCCgQ6AEIWjAQ#v=onepage&q=Dyck%2C%20Harvey%20Leonard.%20Weimar%20Germany%20%26%20Soviet%20Russia%201926-1933%20(1966)&f=false

The "forgotten" story of the links between communism and Nazism.