The most commonly named factors are as follows. The Soviet Union at the time was internationally isolated. Strategically, it was threatened by Japanese expansionism in the east, and German ambitions in the west. Thirdly, Stalin wished to recover territory which had been part of the Russian Empire prior to WWI.

In the 1920s and early 30s, Soviet-German cooperation was based on two things. Exhange of raw materials for processed goods, and exchange of technology. Germany was banned from using many military technologies, such as tanks, and provided the Soviets with technical knowledge in exchange for secret testing grounds. This illustrates how both parties could only turn to each other: Germany was under the Versailles restrictions, and the USSR was internationally isolated. Yet each side to supply the other with neccesary materials.

After Hitler took over, Germany began to (initially) reassert its claims in Eastern Europe. Stalin considered Finland, the Baltic states, Eastern Poland and Bessarabia rightful Soviet territories, as they had been part of the Russian Empire. This is reflected by his expansion into each of these areas by 1940. The Molotov-Ribbentrop pact guaranteed Soviet and German influence in their respectively claimed areas. It therefore appears to be an delineation of spheres of influence between two revanchist states, who were each only threatened by the other in this area of the world. In addition, their mutual support, or at least tacit agreement to the other nation's actions, made condemming or embargoing either country by the international community less effective. The former would require condemnation of both countries, the latter could be circumvented through bilateral trade.

A final note is that in order to rapproach with France, the UK or US, Germany and the Soviet Union would have to recognise and respect the sovereignty of the states created post-WWI. This meant giving up their territorial ambitions, which both were unwilling to do, considering their invasions and occupations of these areas in the period between 1917 and 1945.