The evolution of early strategic planning

In August 1939 the signing of the Nazi–Soviet Non-Aggression Pact (otherwise referred to as the Molotov–Ribbentrop Pact) set out the partition of Poland following an agreed invasion, first by Hitler and later Stalin. The pact heralded a new degree of co-operation between the two nations, while the secret agreement on spheres of influence in eastern Europe and Finland had the important ramification of ensuring security for Germany's eastern border. This freed Hitler's hand for the war he found himself fighting against France and Britain and allowed for the heavy concentration of his forces in the west. Hitler's subsequent campaign in May and June of 1940 seized control of the Low Countries, knocked France out of the war and effectively secured control of Germany's western flank.

It was only at this point that substantial forces were redirected towards the Reich's eastern border, which was the result of both the changed strategic situation in Europe and the events unfolding in the east beyond Germany's frontier. Stalin was moving quickly to absorb his share of the territory allocated to him under the Nazi–Soviet pact, and the remarkable successes Germany achieved in the west added to his sense of urgency. Finland had already been forced to yield territory in the Winter War (1939–40) and by the middle of June the Baltic states were completely occupied and eventually absorbed into the Soviet empire as member republics. June also saw Soviet forces occupying the Romanian regions of Bessarabia and Northern Bukovina, heightening German fears that Soviet ambitions might extend as far as Romania's oil fields at Ploesti, which were vital to Germany's war economy.