Sajer wrote about his experience on the Eastern Front during World War II in his book Le soldat oublié (1965; The Forgotten Soldier). He states that he was an inhabitant of Alsace drafted into the German Wehrmacht at age 16, and that he fought in the elite Großdeutschland Division during World War II. The accuracy and authenticity of the book have been disputed by some historians. Some of the details Sajer mentions are incorrect, while other are impossible to verify due to the lack of surviving witnesses and documents.
The most frequently cited inaccuracy is Sajer's statement that, after being awarded the coveted Grossdeutschland Division cuff title, he and a friend were ordered to sew it on their left sleeves, when it was actually sewn on the right sleeve. Edwin Kennedy wrote that this error was "unimaginable" for a former member of such an elite German unit. Sajer also discusses campaign locations in vague terms and never with specific dates. For example, he asserts that during the summer of 1942 he was briefly assigned to a Luftwaffe training unit in Chemnitz commanded by famed Stuka ace Hans-Ulrich Rudel, but according to Rudel himself, his training unit was actually in Graz, Austria, during the whole of 1942. Sajer mentions seeing "the formidable Focke-Wulf [...] 195s, which could soar up quickly," taking off from an airfield outside Berlin, when no such aircraft ever existed (a Focke-Wulf projekt 195, a heavy transport, was in the pipeline, but never got off the drawing board). Finally, the names of most of Sajer's companions and leaders do not appear on official rolls in the Bundesarchiv, nor are they known to the Grossdeutschland Veterans Association, whose leader, Helmuth Spaeter, was one of the first to question whether Sajer actually served in the Grossdeutschland Division as he claimed.
However, some authors and other Großdeutschland veterans have testified to the book's historical plausibility, even if they cannot speak to the specific events in the book. Lieutenant Hans Joachim Schafmeister-Berckholtz, who served in the Grossdeutschland during the same period as Sajer, confirmed in a letter that he had read the book and considered it an accurate overall account of the Division's battles in the East, while also noting that he remembered a Landser named Sajer in his Panzergrenadier company (5th co), the same company number Sajer mentions being assigned to (though there was more than one "5th Company" in the Division).
Sajer himself struck back against implications of fraud or fiction by claiming that The Forgotten Soldier was intended as a personal narrative, based on his best personal recollections of an intensely chaotic period in German military history, and not an attempt at a serious historical study of World War II: "You ask me questions of chronology, situations, dates, and unimportant details. Historians and archivists have harassed me for a long time with their rude questions. All of this is unimportant. Other authors and high-ranking officers could respond to your questions better than I. I never had the intention to write a historical reference book; rather, I wrote about my innermost emotional experiences as they relate to the events that happened to me in the context of the Second World War." Sajer further stressed the non-technical and anecdotal nature of his book in a 1997 letter to US Army historian Douglas Nash: "Apart from the emotions I brought out, I confess my numerous mistakes. That is why I would like that this book may not be used under [any] circumstances as a strategic or chronological reference."
After reading Sajer's latest letter, one of his staunchest critics, Spaeter of the Grossdeutschland Veteran's Association, recanted his original suspicions of Sajer: "I was deeply impressed by his statements in his letter. ... I have underestimated Herr Sajer and my respect for him has greatly increased. I am myself more of a writer who deals with facts and specifics, much less like one who writes in a literary way. For this reason, I was very skeptical towards the content of his book. I now have greater regard for Herr Sajer and I will read his book once again."
Dutch film director Paul Verhoeven has discussed with Sajer the possibility of turning The Forgotten Soldier into a film.
Sajer has worked extensively in comics published for the Franco-Belgian market under his own name, and also a variety of pseudonyms: "Lahache", "Dimitri Lahache" and just "Dimitri". His comics often include the theme of war and plenty of black humor. He made his debut as a comic artist under the pseudonym "Mouminoux" (his father's surname) in 1946, working extensively in the 1950s and 1960s for the French magazine Cœurs Vaillants and its two spin-offs from Catholic publisher fr:Fleurus presse, creating Jijé inspired comics of a mostly historical nature. For the publisher he co-illustrated two educational books together with younger colleague and future French comic great Jean Giraud, who was also employed at the publisher as comic artist in the period 1956-1958. After a couple of early editorial illustrations, he followed his colleague into the employ of publisher Dargaud in the 1960s, where he created several humoristic comics for its magazine Pilote aside from providing additional illustrations for the editorials.