The Treaty of Versailles in the mind of the Germans stabbed them in the back, especially the guilt clause that made them take blame for the First World War. Hitler capaitalized on these sentiments and used them to rally the German people.
Secondary Sources are materials that digest, analyze, evaluate and interpret information contained within primary sources or other secondary sources. Examples. 1  Autobiographies, memoirs, diaries, emails, oral histories.  Letters, correspondences, eyewitnesses.
A primary source is one that was written/drawn/taken by someone around the time of the event. A secondary source is one that was created some time after the event, by someone who wasn't there but researched it, e.g. a source from a textbook or a historian.