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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.
 Nazi pseudohistory
In 1918, members of the Central Powers began to drop out of the war like flies. The German front line was still into French territory, but the army was running out of reserve forces and was being seriously hurt by blockades, leaving them 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 Russian Revolution was a great victory for Germany. The Spring Offensive had launched them further into French territory. However, German losses had been significantly underreported, as was the 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 institution of the Weimar Republic in 1918 and the Treaty of Versailles in 1919 helped the legend gain traction. To the civilians, it looked as if Germany had been in a position to stalemate at worst and that their sudden defeat was rather suspicious. On top of that, the Treaty of Versailles dumped on them 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 the 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 were a particularly effective scapegoat 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 abolishment of the old 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 peace treaty. Referring to the date of the armistice, politicians from Germany's post-war social democratic government who signed the armistice were dubbed "November criminals."
Adolf Hitler wrote about the stab-in-the-back legend in Mein Kampf 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 the Versailles Treaty. When the Nazis gained control 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 grew forth to some extent: That Germany could 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 was at best wishful thinking on behalf of the defeated German generals — invading Russia has rarely been a good idea.
 Wider usage
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! As you can see, the book's title is overly modest: it's much more than just politically incorrect.
 See also
- 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