On Thursday, April 15, 1920, Victor Kopp, Soviet Russia's special representative sent by Lenin to Berlin, asked at the German Foreign Office whether "there was any possibility of combining the German and the Red Army for a joint war on Poland". This was the start of military cooperation between the two countries, which ended with the German invasion of the Soviet Union on June 22, 1941.

Germany's Army had been reduced to a limit of 100,000 men by the Treaty of Versailles, which also forbade the Germans to have aircraft, tanks, submarines, heavy artillery, poison gas, anti-tank weapons or anti-aircraft guns. A team of inpectors from the League of Nations patrolled German factories and workshops to ensure that these forbidden weapons were not being manufactured.

The Russians offered Germany facilities for building and testing these arms deep inside Russia, well away from inspectors' eyes. In return, the Russians asked for a share in the German technical developments and assistance in creating a Red Army General Staff.

The first German officers went to Russia for these purposes in March 1922. One month later, Junkers began building forbidden aircraft at Fili, Russia , outside Moscow. The great cannon manufacturer Krupp was soon active in south Russia, near Rostov-on-Don. A flying school was established at Vivupal, near Lipetsk, beginning in 1925 to train the first pilots for the Luftwaffe of the future. A tank warfare school was created at Kazan, where the tactics for Blitzkrieg were worked out. This military cooperation enabled the basis for new German armed forces to be created before Hitler came to power on January 30, 1933. A joint German-Soviet poison gas factory in the Urals was not a success.

The Russians offered submarine-building facilities at the Black Sea, but this was not not taken up. The German Navy did take up a later offer of a base near Murmansk, where German vessels could hide from the British. During the Cold War, this base at Polarnoye , built specially for the Germans, became the largest weapons store in the world.

Alongside Moscow's military assistance to the Reich, there was also political backing for Germany's aspirations. On July 19, 1920, Kopp told the German Foreign Office that Russia wanted "a common frontier with Germany, south of Lithuania, approximately on a line with Bialystok". In other words, Poland was to disappear completely. These promptings were repeated over the years, with the Russians always anxious to stress that ideological differences between the two governments were of no account; all that mattered was that the two countries were pursuing the same foreign policy objectives.

All German governments before Hitler rejected any venture into war. On August 23, 1939, Germany and Russia signed a Ribbentrop-Molotov non-aggression pact containing a secret protocol dividing up Eastern Europe. Poland was then indeed divided along the line suggested by Russia nineteen years earlier. Bialystok itself went to Russia, which took the larger share of Poland: 201,000 square kilometres, against 188,500 going to Germany.

While Britain blockaded Germany at sea to prevent her importing war materials from overseas, all the supplies which the Reich needed for the war were sent direct from Russia by rail. Stalin promised that what Russia could not supply from her own resources, she would buy up on the world's markets and pass on to Germany. Three-eighths of the oil used by Germany in 1940 came from Russia, including high-octane spirit for the Luftwaffe to fight the Battle of Britain.

From the start of the war until Germany invaded Russia less than two years later, Stalin had supplied Hitler with 1,5 million tons of oil, the same quantity of grain, and many thousands of tons of rubber, timber, phosphates (for making explosives), iron and many valuable metal ores, particularly chromium, manganese and platinum. At the time of the invasion, Germany was heavily in debt to the Soviet Union.