The 'stab in the back' theory comes from a number of sources. However, it predominantly comes from the association of communism/socialism and Judaism by the right wingers in Germany (and other countries), and the regime change in Germany at the end of the First World War; when German forces were still in enemy territory at the ceasing of hostilities (when the Kaiser was replaced by a left dominated government in the following democracy).

Because of these two points, the right wingers in Germany had their reason for losing the war- the Jews, socialists and communists had colluded to betray the fighting men at the front, and seize power within Germany. To really grasp this, one must be aware that the Reichstag had been at odds with the Kaiser for many years before the war, and the thing the ruling class in Germany feared most was a rise of the socialists (as this would invariably lead to the fall of their ruling class status). When the war ended, the nationalists and monarchists had their excuse for both of the injustices placed upon them- the loss of the war and the fall of the Kaiser.