Aufbau Ost (German: Building in the East) was the German operational code name for the mobilisation of forces prior to the commencement of Operation Barbarossa and the subsequent invasion of the Soviet Union.
On August 8, 1940 Hitler ordered Walter Warlimont, Deputy Chief of Alfred Jodl, to determine the positions of Soviet troops in the East. The appropriate directive was signed the next day by Wilhelm Keitel. It stated that due to the threat of British air attack on Eastern Germany it was necessary to use the eastern territories for drawing and preparing the new units. In compliance with the directive, the building organization of Fritz Todt started equipping the battleground while the rear organs managed the logistical support in the East. Meanwhile preparations for adjusting the Soviet rail gauge to match the Western European were taking place.