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 Source vs Secondary Sources in the Sciences. In the Sciences, primary sources are documents that provide full description of the original research. For example, a primary source would be a journal article where scientists describe their research on the human immune systems. A secondary source would be an article commenting on or analyzing the scientists' research on the human immune system.