On 7 May 1939, Blumentritt submitted to his superiors a working plan for the German Invasion of Poland. At this time, he was assigned to the "Oberkommando des Heeres" (Army High Command; OKH), and formed along with Manstein and "Generalfeldmarshall" Gerd von Rundstedt a "Working Staff" for the development of a plan for the invasion. The plan he submitted was called "Fall Weiss" (Case White) and was subsequently put into action with little alteration. Then on 2 September 1939, Blumentritt was transferred to the general staff of Army Group South (one of the two German Army Groups to carry out the invasion) in Silesia under Rundstedt. This collaboration began his long and intimate friendship with Rundstedt, which was to last for many years. Blumentritt was Rundstedt's Chief of Operations, while Manstein was Chief of Staff. The invasion was put into motion, after some delays, on 1 September 1939, and by 6 October the entire country of Poland was subdued.