biography of Frederick the Great has been described by the American military historian Peter Paret as one of the finest military biographies ever written. The historian Russell Weigley called it "the best introduction to Frederick the Great and indeed to European warfare in his time". James J. Sheehan says it is the best book in English on the famous king. Ritter's biography was designed as a challenge to Nazi ideology, which claimed a continuity between Frederick and Hitler. Dorpalen notef, "The book was indeed a very courageous indictment of Hitler's irrationalism and recklessness, his ideological fanaticism and insatiable lust for power". Dorpalen, nevertheless, criticised Ritter's historiography as apologetic of Prussian militarism, German past and figures like Frederick the Great and Bismarck Ritter's emphasis on Frederick's limited war aims and willingness to settle for less than he initially sought was seen at the time as a form of oblique criticism of Adolf Hitler. In addition, the emphasis that Ritter placed on the influence of the Enlightenment and "orderly reason" on Fredrick were intended by Ritter to disprove Hitler's claim quietly of being Frederick's successor. The inspiration behind the Fredrick biography was Ritter's personal reaction to the Day of Potsdam, 22 March 1933, on which Hitler had laid claim to the Prussian traditions in a way that Ritter felt was not historically accurate. In March 1936, upon witnessing the remilitarization of the Rhineland, Ritter wrote in a letter to his mother that for his children "who had never seen German soldiers from close up, this is one of the greatest experiences ever.... Truly a great and magnificent experience. May God grant that it does not lead to some international catastrophe". Acts against regime Ritter was a devout Lutheran and became a member of the Confessing Church, a group of dissenting Lutherans that resisted the Nazi-inspired and Nazi-imposed "Aryan Christianity" during the 1930s. In 1938, Ritter was the only faculty member at Freiburg to attend the funeral of Edmund Husserl, who was considered the founder of the modern philosophical school of phenomenology. Husserl had been on the faculty at the University of Freiburg until the Nazis in 1933 caused him to be dismissed because of his Jewish origins. Husserl was then also prevented from publishing his works. Ritter's presence at the funeral of Husserl has been widely interpreted ever since as an act of quiet courage and political protest against the Nazi regime.