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.
1 Secondary sources may draw on primary sources and other secondary sources to create a general overview; or to make analytic or synthetic claims.  Tertiary sources are publications such as encyclopedias or other compendia that sum up secondary and primary sources. For example, Wikipedia itself is a tertiary source.