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The stab-in-the-back legend (Dolchstoßlegende in German) was a conspiracy theory propagated by the Nazis and other right-wing groups in the aftermath of World War I that blamed the Bolsheviks, Weimar politicians, and Jews for the loss of the war.
In 1918, members of the Central Powers began to drop out of the war like flies. On the Western Front, the German lines still ran over French territory, but the Imperial army had started to run out of reserve forces and to suffer seriously from blockades, leaving it chronically short of nearly every war material and even food. Additionally, the arrival of fresh American troops led to a very successful Allied "Hundred Days Offensive", which was rapidly pushing German forces out of France. While the tide had turned against Germany at this point, the picture was entirely different for German civilians. The withdrawal of Russia in 1917 as a result of the October Revolution represented a great victory for Germany. The Spring Offensive had launched German forces further into French territory. However, German losses had been significantly under-reported, as was the ultimate failure of the Spring Offensive and launching of the Hundred Days Offensive, making the German surrender seem like it had come from nowhere to those on the home front.
General Erich Ludendorff is generally credited with coining the phrase stab-in-the-back, or at least popularizing the concept, upon returning home in 1918. The establishment of the Weimar Republic in November 1918 and the Treaty of Versailles in 1919 helped the legend gain traction. To German civilians, it looked as if Germany had been in a position to stalemate at worst, and made their sudden defeat seem rather suspicious. On top of that, the Treaty of Versailles dumped on Germans very punitive war reparations as well as blaming Germany entirely for the war. Like Ludendorff, many Germans went into denial. Isolated incidents of union strikes at munitions factories, desertions, and mutinies were exaggerated and contorted into a grand conspiracy of internal dissidents. Various theories sprung up, generally making assertions that Bolshevik unions had helped cut off supplies to the front lines, and that the commies, Jews, and Weimar republicans had secretly worked to incite internal dissent in the military, cut back-room deals with the Allied Powers, and generally sold out Germany for their own gain. The theories found considerable acceptance among nationalists and anti-Semites. The Jews made particularly effective scapegoats due to Germany's history of anti-Semitism. The legend also proved effective at delegitimizing the newly-established democracy: while the aristocratic military elites had been responsible for Germany's debacle, the abolition of the old Second Reich system meant they could step into the background and let the new democratic government take full responsibility for the punitive terms of the armistice and of the peace treaty. Referring to the date of the armistice, opponents of the politicians from Germany's post-war social democratic government (who had signed the armistice) dubbed them the "November criminals".
Adolf Hitler wrote about the stab-in-the-back legend in Mein Kampf (first published in 1925) fantasizing about a shadowy alliance between Bolsheviks, center-left Weimar politicians, and Jews. Nazi propaganda made heavy use of the legend, which exploited German resentment of the Allied Powers and of the Versailles Treaty. When the Nazis gained control of Germany's government in 1933, the party shoved accounts of historical reality down the memory hole and promoted the negationist account to the "official story".
After World War II a new "stab-in-the-back" legend flourished to some extent: Germany could allegedly have won the war, had it not been for the incompetent leadership of Hitler and the Nazis. While having a mentally unstable ex-artist as commander-in-chief did not help matters, this demonstrated at best wishful thinking on behalf of the defeated German generals — invading Russia has rarely proven a good idea.
Blaming internal dissidents for the loss of a war is a recurring theme in history. Thus, political commentators sometimes refer to these legends of betrayal as "stab-in-the-back" theories. Similar beliefs in France about the Algerian War led to a right-wing terrorist campaign against the government through the early '60s. One example from America is the wingnut idea that the US would have won the Vietnam War if it weren't for the dirty hippies and liberal media destroying our troops' morale. If we had just sung the national anthem loud enough and waved our flags hard enough, victory could have been ours! Ultra-batshit publishing house Regnery Publishing manages to one-up this theory with its Politically Incorrect Guide to Vietnam, which claims that the US actually did win and the true history is just being covered up by liberal history professors! Apparently, the liberal conspiracy is so strong it was able to cause South Vietnam to be overrun, and the communist party to take control of the entire country. As you can see, the book's title is overly modest: it's much more than just politically incorrect. However, this myth is widespread in popular discourse, with the idea of the spat upon Vietnam vet being a common meme, despite the fact that there are exactly zero confirmed cases of it ever happening.
Another example of the legend can be found south of the border. After the disastrous Mexican-American War the Catholic Church became the target of some backstab legends. According to this theories, had the Catholic Church lent their enormous resources to the government, the Mexican army would've had a chance to defeat the invaders. Some people even go as far as claiming that the Church struck a deal with the Americans; US soldiers would not harm any of their properties and they in turn would tell the deeply religious people that killing Americans was a sin, thus limiting the recruitment abilities of the army.
- Lost Cause of the South (for the neo-Confederate exercise in pseudohistorical revisionism and scapegoating)
- Holocaust denial (denial of one of the ultimate consequences of the accusations of the stab-in-the-back legend and typically peddled by the legend's "intellectual" successors)
- Stab-in-the-back propaganda, and a book review