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.
Primary sources are the first hand evidence left behind by participants or observers at the time of events. Primary sources originate in the time period that historians are studying. They vary a great deal.
Primary sources are written by the scientists that performed the experiment and are writing about it. They are normally found in peer-reviewed journals. Your example is of a primary source. Primary sources can cite other articles, but must be based on novel research. Secondary sources are commonly found in general magazines and newspapers in the science sections. They cite original research, and contribute nothing new or original to the report.