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.
In the natural and social sciences, the results of an experiment or study are typically found in scholarly articles or papers delivered at conferences, so those articles and papers that present the original results are considered primary sources. A secondary source is something written about a primary source. Secondary sources include comments on, interpretations of, or discussions about the original material.