a very specific contribution from Russians and Germans.
If you don't know sh*it, it's better to keep quiet. This way others won't notice you're a dumbfuck.
Cooperation between Soviet Union and Germany dates back to Treaty of Rapallo (1922), which included military matters. Germans were able to keep developing their armored weaponry and tactics thanks to access to Soviet training grounds.
And Ribbentrop-Molotov treaty. Secret protocol to the treaty not only stipulated the partition of Poland (obvious military joint action), but also division of the remaining countries of the region, with Soviet Union getting the Baltic States (with the exception of Klaipeda, which was ceded to Germany) and a large chunk of Romania.
And during the years 1939-41 Soviet Union provided Germany not only with large amounts of war materiel but also with... naval bases for U-bots operating in North Atlantic.