Treaty of Versailles

From the Handbook for Schooling the Hitler Youth

Chapter Ten: The Political Area of the German Folk (Handbook online is here)

The Rhine, Danube (running West to East from the Rhine all the way to the Black Sea), and the Vistula were considered the life arteries of the historic German Reich.

The following has been edited for greater brevity -cy

The boundaries of the German Reich - the political territory of our people – have experienced many changes since the beginning of German history. During periods of unity of political purpose, the frontiers of the state extended beyond the borders of the German population area. Periods of internal dissension and lack of unity, however, led to the involution of the state's territory behind the population area.