In 1918, Germany had lost the war. The German head quarter asked the civilian government to ask an armistice to the allied powers.
But the junkers and other high ranking officiers could not admit that they had lost, because of their incompetence and mistakes.
As the allied did not invade Germany, the German soldiers came home with weapons and paraded in the street.
The German generals and the nationalist leaders explained that the war was not lost but that Germany and its army were betrayed by the civilian government, by the leftists and by the Jews. This idea was one of the main idea which helped the Nazis to come to power.
[Stab-in-the-back myth - Wikipedia]
From Quora:
The “stab-in-the-back myth” (in German “Dolchstoßlegende”) was a reactionary conspiracy myth about Germany’s defeat in World War One in 1918.
The basic claim was that the German military did not lose the war, that it was “stabbed in the back” by leftist revolutionaries who overthrew the government, abolished the monarchy and created the democratic Weimar Republic. It was also frequently combined with blaming the democratic politicians for accepting the unfavorable conditions of the Treaty of Versailles, sometimes also with the claim that it was all a preplanned Jewish plot. That last bit should give you a good idea with whom this myth was particularly popular.
It was completely counterfactual nonsense, invented by cowardly high-ranking military leaders in order to deflect blame from themselves for the military defeat they suffered (in good part through incompetence).
In reality:
- In 1918, all allies of Germany had surrendered or sued for peace, the German military was beaten and a defeat unavoidable.
- The revolution started with soldiers, specifically sailors of the High Seas Fleet who went into mutiny when ordered to prepare for a battle that was clearly a pointless suicide mission.
- The Treaty of Versailles was somewhat harsh, but Germany really didn’t have any choice about accepting it. The conditions would only have been worse if Germany had fought on longer.