Hassel's biography is disputed (see below). He claims that he was born as Sven Pedersen at Frederiksborg, Zealand, Denmark. He later adopted his mother's maiden name Hassel. At the age of 14 he joined a merchant navy as a cabin boy and worked in ship until his military service in 1936. In 1937, to escape the Great Depression, the unemployed Hassel moved to Germany to join the army. In an interview in 1990, he said, "Germany happened to be closer than England, I went to a Wehrmacht recruiting office to enlist, but it wasn't as easy as I had thought. Only German citizens could serve. After six months of trying to join up, the Seventh Cavalry Regiment finally accepted me on the condition that I became a naturalized German." Later he served with the 2nd Panzer Division stationed at Eisenach and in 1939 was a tank driver during the invasion of Poland.

A year later he attempted to escape. He served with the 2nd Cavalry Regiment and later the 11th and 27th Panzer Regiments (6th Panzer Division) on all fronts except North Africa and was wounded several times. Eventually he reached the rank of lieutenant and received an Iron Cross 1st and 2nd class. He surrendered to Soviet troops in Berlin in 1945 and spent the following years in various POW camps. He began to write his first book, Legion of the Damned while he was interned.

He was released in 1949, and was planning to join the French Foreign Legion when he met Dorthe Jensen, whom he married in 1951. He went to work in a car factory, but his wife encouraged him to continue to write about his experiences. De Fordømtes Legion (Legion of the Damned) was published in 1953.

In 1957 Sven Hassel suffered from an attack of a sickness caught during the war and was paralyzed for almost two years. After recovery, he began to write more books. In 1964 he moved to Barcelona, Spain, where, as of 2009, he still lives. In total he has published fourteen novels which have been translated into eighteen languages. In 1987 his book Døden på larvefødder (Wheels of Terror) was made into a film with the title The Misfit Brigade.

Hassel's books are written in the first person, with Hassel himself as a character, though not necessarily the lead character. The books describe the exploits of a 27th (Penal) Panzer Regiment composed of expendable soldiers - sentenced criminals, court-martialed soldiers and political undesirables. In addition to Sven, they include Alfred Kalb, "Legionnaire" (ex-member of French Foreign Legion); Wolfgang Creutzfeldt, a giant of a man ironically named Tiny (variously Little John in some of the books); barracks fixer and shrewd thief Joseph Porta; older sergeant Willie Beier, "Old Un" or "Old Man"; Julius Heide, an ex-waffen SS officer and Nazi fanatic, Barcelona Blom, a veteran of both sides of the Spanish Civil War, Gregor Martin, who was a removals man before the war, Chief Mechanic Wolf, and Staff Sergeant Hoffman, a typical German NCO of the period. They serve on most fronts from Northern Finland to the Russian Front (more than once), Italy (Monte Cassino), Greece (The Bloody Road to Death) and the Balkans, and to Normandy (Liquidate Paris) during the Normandy Invasion. Wheels of Terror contains some moving chapters on the bombing of Hamburg by the RAF. SS-General describes the Battle of Stalingrad in some detail and includes a description of a breakout of German troops from the encirclement led by the titular S.S General. Reign of Hell describes the destruction of Warsaw in the final months of the Second World War when the regiment appear to be fighting in Poland. Wheels of Terror also describes many incidents of atrocity and torture carried out by both German and Russian troops.

Whilst most of the action appears to occur in Russia, a quick chronological analysis of the activities described in the books appears to show that the regiment depicted in the books fought in several places, hundreds of miles apart, at the same time. In some of the books the 27th Regiment does guard duty for the Gestapo in Hamburg (Assignment Gestapo) and also at the military prison at Torgau (March Battalion). Hassel states that the characters are based on real people and events are related to historical events.

Many sections of the books are based around the characters' reminiscences of pre-war life, both civilian and military, or of their exploits (mostly involving drunkenness and petty crime) when out of action. The later books are considerably less realistic than the earlier ones, with the characters involved in improbable criminal activity behind enemy lines and increasingly bizarre story lines. Although the violence is just as intense as the earlier novels, the plot of books such as The Commisar, Execution and OGPU Prison owes more to the active imagination of the author than to any realistic account of wartime action, but, like the other works, including a strong streak of black comedy.

Hassel's view of war is brutal. In his books, soldiers fight only to survive, the Geneva Convention being a dead letter to all sides. People are killed by chance or with very little reason. Occasional pleasant events and peaceful meetings are brutally cut short. Unsympathetic Prussian officers constantly threaten their men with courts-martial and execute them with little provocation. Disgruntled soldiers occasionally kill their own officers to get rid of them. By graphically portraying war as violent and hopeless in such manner, Sven Hassel's books have been said to contain an anti-war message. His first book Legion of the Damned has been compared to a much grislier, darker more terrifing version of Erich Maria Remarque's All Quiet on the Western Front.

Even though Hassel's books are not taken at all seriously by historians or military professionals, some of their background material at least appears to be derived from authentic accounts.

The Danish writer Erik Haaest has been disputing Hassel's claims for many years.

Haaest writes that Sven Hassel is actually Børge Villy Redsted Pedersen, a Danish Nazi who never served on the Russian front. According to Haaest, the author spent the majority of World War II in occupied Denmark and his knowledge of warfare comes second-hand from Danish Waffen SS veterans whom he met after the end of the war. Haaest also alleges that Hassel's first novel was ghostwritten and when it became a success, he employed his wife to write the rest of his books.

Although Haaest's allegations are not generally accepted by Sven Hassel fans, they have focused attention on Hassel's description of his own past and provoked much discussion, particularly on Internet newsgroups and discussion forums, on the validity of Hassel's claims.

In 2007 the Danish Arts Council was criticized for providing funding to Haaest for research into Danes who served in the SS, on the grounds that Haaest had stated that the concentration camp gas chambers never existed and that the Diary of Anne Frank was a forgery.

Haaest claims that during the war period, Pedersen/Hassel, was in fact a member of the HIPO Corps or Hilfspolizei, an auxiliary Danish police force created by the Gestapo, consisting of collaborators. According to Haaest, Pedersen aka Hassel was actually put on trial in Denmark, but avoided a death sentence – a fate that many HIPO members faced.

Others claim that Hassel was indeed a member of the Waffen-SS who covered up his wartime activities by claiming to have served in a penal battalion. Some passages seem very authentic, the book Wheels of Terror contains some very detailed accounts about the German breakout from the Korsun-Cherkassy Pocket. The fighting at Novaya Buda (which Hassel calls "Nova Buda"), as well as the breakout at Lysyanka, are dealt with from a survivor's point of view. Wheels of Terror also involves a battle for a railroad junction like that of Kovel. It so happens that the fighting in all of the above mentioned areas, involved Danish Waffen SS volunteers from the 5th SS Panzer Division Wiking. Later scenes of fighting in Wheels of Terror resemble the Battle of Narva, which included Danish Waffen SS volunteers from the Nordland.

This lends some credence to some assertion that Hassel, gleaned some of the material in his books from either his own experiences or from friends/cellmates who were Danish Waffen SS volunteers.

While Haaest has been instrumental in casting public doubt on Sven Hassel's stories, they were never taken seriously by military or history professionals to begin with. For example, there was a 27th regiment in the German Army, but it was not penal. Tiger I tanks were in short supply, and were organized in special battalions, directly under Corps command, but attached to a few elite divisions; they certainly were not given to any penal formation as shown in Hassel's books.

The readership for Sven Hassel has spread to many countries. Although the popularity of his novels peaked in the 1970s and early 1980s, the advent of the Internet has sparked a new interest - and an opportunity for older fans to exchange points of view.

In 1988 a humour book called The Book of Revelations (Mark Leigh and Mike Lepine) printed a parody of both Beatrix Potter and Sven Hassel entitled "Peter Rabbit - Tank Killer" - lampooning the fashion of the period for unscrupulous publishers and literary estates to draft in famous authors to pen "continuations" of their most successful characters and series (the most notorious being that of William Horwood's much criticised continuation of Kenneth Grahame's Wind in the Willows).

Although the book is out of print, copies of Peter Rabbit - Tank Killer are still to be found all over the net - particularly any sites concerning computer games based on World War Two and the various war re-enactment societies. The occasional T-Shirt of the book's "cover" also turns up at re-enactment events.

In the "Flood" episode of the 1980s BBC situation comedy The Young Ones, Vyvyan admits to reading a comic entitled "SS Death Camp Criminal Battalion go to Monte Cassino for the Massacre".

(English names)

• Legion of the Damned
• Wheels Of Terror
• Comrades of War
• March Battalion
• Assignment Gestapo
• Monte Cassino (The Beast Regiment)
• Liquidate Paris
• SS-General
• Reign of Hell ISBN 0304366900
• Blitzfreeze
• The Bloody Road to Death
• Court Martial
• O.G.P.U. Prison
• The Commissar