Aufbau Ost was the German

Nazi Germany

Nazi Germany , also known as the Third Reich , but officially called German Reich from 1933 to 1943 and Greater German Reich from 26 June 1943 onward, is the name commonly used to refer to the state of Germany from 1933 to 1945, when it was a totalitarian dictatorship ruled by...

 operational code name for the mobilisation of forces prior to the commencement of Operation Barbarossa

Operation Barbarossa

Operation Barbarossa was the code name for Germany's invasion of the Soviet Union during World War II that began on 22 June 1941. Over 4.5 million troops of the Axis powers invaded the USSR along a front., the largest invasion in the history of warfare...

 and the subsequent invasion of the Soviet Union.

On August 8, 1940 Hitler ordered Walter Warlimont

Walter Warlimont

Walter Warlimont was a German officer known for his role in the OKW inner circle .-World War I:...

, Deputy Chief of Alfred Jodl

Alfred Jodl

Alfred Josef Ferdinand Jodl was a German military commander, attaining the position of Chief of the Operations Staff of the Armed Forces High Command during World War II, acting as deputy to Wilhelm Keitel...

, to determine the positions of Soviet troops in the East. The appropriate directive was signed the next day by Wilhelm Keitel

Wilhelm Keitel

Wilhelm Bodewin Gustav Keitel was a German field marshal . As head of the Oberkommando der Wehrmacht and de facto war minister, he was one of Germany's most senior military leaders during World War II...

. 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

Fritz Todt

Fritz Todt was a German engineer and senior Nazi figure, the founder of Organisation Todt. He died in a plane crash during World War II.- Life :Todt was born in Pforzheim to a father who owned a small factory...

 started equipping the battleground while the rear organs managed the logistical support in the East. Meanwhile preparations for adjusting the Soviet rail gauge

Rail gauge

Track gauge or rail gauge is the distance between the inner sides of the heads of the two load bearing rails that make up a single railway line. Sixty percent of the world's railways use a standard gauge of . Wider gauges are called broad gauge; smaller gauges, narrow gauge. Break-of-gauge refers...

to match the Western European were taking place.