After WWI General Ludendorff began a second career as an ultra-right politican and writer.
During that time he wrote book called "Weltkrieg droht" describing in great detail a possible future world war II.
I came across it by accident in the state library of Württemberg some years ago and realised that it is now avaiable
on the net. (has some maps for those not understanding german)
He basically expected WWI style combat + large scale partisan war + massive genocides (with the Germans, still having their 100 000 men Weimar army, as the main victims).
At first France and its minor allies (Poland, Czechosolovacia, ...) attack Germany, which the UK supports with an expeditionary force landing at Hamburg.
The Germans get crushed and retreat to the Harz and the Baltic Coast (their civilians starving by the millions).
Then the Soviet Union enters the war on the Anglo-German side and annhilates Poland while France is occupied in Germany.
Later on socialist revolutions occur in Italy and Spain.
The african & asian colonies revolt and butcher all europeans, while different groups try to pull the Americas into the war on their side.
Then civilisation collapses...
This is mixed with conspiracy theories that make Hitler look sane (the war is in truth a fight for world domination between mansons, communists and the Vatican).