True, the Germans ground down the Russian armies through the Great War and finally got their reward in the Treaty of Brest-Litovsk, which should (amongst other things) have given them access to vast amounts of wheat, coal etc. But, it took so much of the crappier half of the German army to manage the occupation that they were short of troops in the decisive western theatre. And a lot of the divisions dragged back from Russia after mid-1917 were infested with communist rot, which they gave to anyone willing and interested.I think that Montgomery was pretty disingenuous when he said that the first rule of warfare was don't invade Russia. Germany had successfully invaded Russia in the winter of 1917/18 and had knocked them out of the war. Considering this, and the Soviet army's performance since 1917 it is not surprising that the Germans thought that it was doable.