What proof did the Nazis offer that the Jews were responsible for Germany's economic collapse after WWI?

One of the primary items they used was a publication called "The Protocols of the Learned Elders of Zion", a forgery that purported to document a meeting of Jews in Geneva in 1897 in which they planned world domination. The first German translation of the "Protocols" was published in 1919.

This document had a tremendous impact on the Nazi leadership. Documents indicate that some of the senior Nazi officials, such as Hitler, Rosenberg, and the cretinous Jew-baiter Streicher (publisher of "Der Sturmer", a newspaper used to disseminate Nazi party propaganda), believed this publication to be true. It is known to have strongly influenced Goebbels, whether or not he believed it valid.

"The Protocols of the Learned Elders of Zion" was created to influence Tzar Nicholas II of Russia during and following the first Russian revolution in 1905. The original author has been identified as Mathieu Golovinski, a Russian exile living in Paris and working for the Okhrana, the Russian secret police. It was first published as part of the "The Great and the Small" by Sergius Nilus, a rabid anti-semite. Golovinski copied the "Protocols" in large part from a book published in 1864 by Maurice Joly, "The Dialogue in Hell Between Machiavelli and Montesquieu", an attack on the French emperor Napoleon III.

The Tzar, a weak and ineffective ruler, was convinced of a Jewish plot by this tract and by his conservative advisors, who were opposed to liberal modernizations. A series of vicious pogroms were subsequently launched against Russian Jews. The political situation within Russia continued to deteriorate and, ultimately, led to the success of the second revolution in 1917.

The Jews have been a convenient group to blame for the ills of the world. There are enough of them in western society and they occupy all the strata in the socio-economic ladder, making them convenient scapegoats for people seeking illegitimate power.

The "Protocols" have been called a vampire that will not die and continue to be published to this day, after having been proven a forgery as early as 1920.