The word ‘allegedly’ is used advisedly here – there is much debate as to how autobiographical his novels actually were, with the concensus tending towards ‘not at all’. It is rumoured that he spent his war in Copenhagen (he was a Dane) stealing bicycles and informing for the local Nazi party.
Monte Cassino is a case in point. Hassell writes of his part in the famous battle and of his wider experiences in the Italian front late in the war and his story is improbable to say the least. I won’t elaborate with details, suffice to say that it reads like the story that you’d get from someone who /wasn’t/ there but desperately wants you to think that he /was/.
His exploits, and those of his comrades-in-arms – Porta, The Old Man, Tiny, Barcelona Blom and The Legionnaire – verge on (and frequently exceed) the frankly fantastical. There are various factual inaccuracies that don’t help his case; for instance he claims to have fought the US Marines on the beaches of Anzio (the USMC never fought in the European war) and to have been dive-bombed by four-engined heavy bombers.
There is room for doubt, however, the battle for the monastery /was/ savage in the extreme and was fought many years ago, so a benevolent reader could forgive Hassell his mistakes and exaggeration and still believe the substance. However, taken with his full – and far from insubstantial – body of work, it stretches the imagination that he could have fought in Greece, the Russian Front, Stalingrad, Monte Casino, Northern and central Europe, and been incarcerated in various Russian and German concentration camps and survived to write about it (he died in 2012, having penned 14 books).