First World War gave him a great deal of respect for the Russian soldiers. He maintained this respect throughout his career, and regretted that many of his fellow officers, with less experience in the East, did not share it. He said of the Russians, "... in defense the Imperial Russian Army was stubborn and tenacious and they were masters at constructing defensive positions with great speed. The Russian soldier showed great skill in night operations and in forest fighting, and he preferred hand-to-hand combat. His physical needs were slight and his ability to stand up to punishment unshaken truly astounding." Later, during the interwar period Blumentritt served as a company leader in the 3rd Thuringian Infantry Regiment No. 71 from 20 February 1919. After the military restrictions imposed by the Treaty of Versailles came into effect, he was briefly a member of the Freikorps (paramilitary organization) formed by the veterans of the 3rd Thuringian, before transferring back to the regular army with the 22nd Reichswehr Rifle Regiment on 1 October 1919. Then later he served as a staff officer in several positions, first as operations command officer from 1 April 1926 in the staff of the 6th Division. He was promoted to major in September, 1933, then worked as a lecturer and tactics instructor at the Kriegsakademie in 1935, and finally was promoted to oberst in October 1938. He eventually served as a staff officer under Wilhelm Ritter von Leeb, along with his friend Erich von Manstein. Second World War German invasions 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,