I'll have a go at this.

It's worth remembering that the end of the First World War was not like the second: the British and French were near the end of their resources in 1918, calling up very young men and greatly concerned about how far their resources could stretch. Even in August 1918 most authorities in London, including the Chief of the Imperial General Staff and the Prime Minister, were preparing for what they hoped would be the decisive campaign in summer 1919. Germany's resources were (as we recognise now) exhausted, but to the Allies it still looked like an impregnable monolith squarely behind its imperial ruler and the military dictatorship behind it. With hindsight we know Germany was about to fall apart, but this was far from clear at the time and the Allies had spent four years before March 1918 assaulting the German lines with very little success, and this was a far-fetched concept.

Secondly, there was no sympathy for Germany. The French had seen a large swathe of their country destroyed, and that after losing Alsace-Lorraine in the 1871 war. German occupation was harsh, with executions or 'spies', removal of men for work in Germany, and towns near the front lines 'emptied' and the 'useless mouths' forcibly evacuated back to France via neutral counties. Food was requisitioned and any form of poaching forbidden. To be found with a gun in your possession often meant execution, even if it was a militarily useless piece.

The U-boat war was waged overtly against any ship, again unprecedented, as attackers took no responsibility for their victims. The Germans had initiated the bombing of British and French cities, and built the Paris Gun specifically to terrorise the French capital.

While this doesn't look so bad after the following war, at the time it was unprecedented. The treaties imposed on Russia and Romania were draconian and worse than Versailles.

So what all this meant was a very limited hearing for those who wanted a constructive peace with Germany, even if there were not millions of British and French families with dead sons, fathers and husbands from a war that by 1918 looked to have no cause but to send the Germans back to Germany. The Allies were exhausted, and it's no wonder the average British or French voter did not care at all to ameliorate the impact of defeat on the German opposites.

Now having said all that, the 'stab in the back' theory was fairly marginal until the Depression, over ten years after the war, although the ingredients were there in 1919. In the late 1920s the Weimar Republic had driven historic militarism and authoritarianism to the edges of German politics. The Freikorps operated to prevent a socialist revolution, something the Allies were probably mostly quite happy to see, but it was just a memory by the mid 1920s.

You make a good point that the Allies did not make clear the role of the German Government and the reality of the military defeat of Germany. But as per my opening point, the collapse of Germany was something of a surprise to the Allies, and the experience of war had caused a deep, lasting anti-militarism to permeated British and French society, while in the US many people felt they had been tricked into joining the war by British propaganda. While victory was celebrated, there was no deeper pride in a victory that had literally killed and maimed a generation of young men and scarred almost every family in every town. While some were aware of and proud of the defeat inflicted on Germany, the losses were so massive and the end so sudden that this was not really well understood.

Finally, the militarists who controlled Germany, the Kaiser, Hindenburg and Ludendorff, were quite cunning in the way they ended the war, handing power to the Reichstag only when defeat was completely inevitable. Thus, a left-leaning democratic institution got to be responsible for going cap-in-hand to the vengeful Allies to seek peace, while the dictatorship had hidden from the German people the true nature of their situation. The average German thought the British were starving due to the U-boats, and that their brave army was fighting stoically. Ludendorff demanded peace be made before the Allies got onto German soil, so the Germans never saw triumphant Allied armies on parade, nor their cities in ruins (two errors not repeated in 1945). Few saw their mutinous soldiers, and those that did were in denial by the 1930s. It's very hard to use mere facts to counter such realities, we suffer endless fake news even in today's much more (supposedly) sophisticated environment.

The militarists just had to wait for the civilian government to falter for their comforting lie to gain traction.

It might be worth remembering what a great job the Allies did in 1945 to completely crush militarism and ultra-nationalism in the nations they defeated: as we comb for supposed Allied war crimes, we often forget this I think.

AJP Taylor The Causes of World War II

Nicholas Wapshott Keynes vs Hayek

Helen McPhail St Quentin

John Terraine To Win A War: 1918 The Year of Victory