'Aufbau Ost' was the German programme for the gradual eastward shift of troops, weapons, equipment and supplies into occupied Poland in preparation for the launch of 'Barbarossa' (spring/summer 1941).
The undertaking was implemented with great care lest the Soviets be alarmed by the sudden appearance of vastly increased numbers of men and quantities of matériel and war supplies. The secrecy of the movement was made the more important as the Soviets knew that German aircraft were overflying the western USSR and were likely to be gathering tactical and operational reconnaissance vital for the success of imminent German land operations.
It was on 8 August 1940 that Adolf Hitler ordered Generalmajor Walter Warlimont, deputy chief of operations in the Oberkommando der Wehrmacht, and as such the deputy of General Alfred Jodl, the OKW chief of operations, to determine the location of Soviet troops in the western part of the USSR. The associated directive was issued on the following day by Generalfeldmarschall Wilhelm Keitel, head of the OKW. The directive stated that as a result of the threat of British air attack on targets in eastern Germany, it was necessary to use the 'eastern territories' (German-occupied western Poland) for the establishment and training of new formations. In further compliance with the directive the Organisation 'Todt', the Nazi labour organisation established and run by Fritz Todt, started to develop the logistical infrastructure of western Poland, and preparations were started for the adjustment of the Soviet rail gauge to match that of western Europe.