How 'accurate' is historical fiction?

Salve,
today we will have a geust - a British writer Ian Ross, autor of 2 historical novels - see Ian's Amazon page.

Subejct matter is straighforward, or is it? I read  three writers'  novels mentioned in the essay Alfred Duggan, Rosemary Sutcliff, Wallace Breem, and I especially enjoyed Alfred Duggan's historical fiction. I have not read George Shipway, got to fix this omission (wiki claims he wrote about cavalry combat in detail,  - :) )

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How 'accurate' is historical fiction?
Novelists who write about the past are often asked about the importance of historical accuracy in their work. This is perhaps a strange question; history, after all, is not an exact science. The past no longer exists, so how could we measure the accuracy of our view of it? Instead, history is a method of attempting to understand the fragments left to us of the past, a set of tools and parameters for interpretation and speculation.
But, of course, this isn’t really want the question is about. ‘Accuracy’ (for want of a better word) in historical fiction is all about accordance with the sources, paying attention to details and not veering off into fantasy. It is about the construction of a plausible view of the past that fits with what we know and does not contain jarring anachronisms.
Put like this, the question is much easier to answer, for me at least: ‘accuracy’ is extremely important. One of the most fascinating aspects of historical fiction is the constant collision and interplay between the novelistic imagination and the raw matter of the past. Individual stories take root from the greater story of past events, and are constantly fed by it. Beyond the story itself, the structure that will get the characters from prologue to dénouement and hopefully carry the readers along with it, there is the accumulation of supporting details.
Historical research provides the furniture of my character’s world, the clothes they wear and the food they eat. It provides the thoughts in their heads. It is a liberation, not a chore. The more I know of the period I’m writing about, the more comfortable and confident I feel about imagining the bits I don’t know. And, of course, it’s those gulfs of the unknown, and the bridges we build to cross them, that makes the exercise so rewarding.
But can we take ‘historical accuracy’ too far? In this age of the internet, the raw matter of history is available to all, the sources and the speculations about any era easily accessible. So should novelists spend less time worrying about ‘accuracy’, and more on telling a unique and engaging story?
Writers of a previous generation were certainly less concerned about historical rivet-counting. Rosemary Sutcliff’s Roman novels are powerfully evocative works of imagination, but at times could have historical purists wincing. Wallace Breem’s novelEagle in the Snow concerns the fall of the Rhine frontier in the 5th century, but his Roman army seems largely transplanted from the days of Hadrian. Both Alfred Duggan and George Shipway wrote novels of the Roman past which have stood the test of time, although by the standards of modern scholarship they may default on the details.
Breem’s portrayal of the barbarian hordes menacing the Roman frontier, however, may have been inspired by own experiences with the North West Frontier Force in British India. Shipway’s painstaking narration of Suetonius Paulinus’s army on the march through Britain on the eve of Boudica’s revolt perhaps draws on his time as a cavalry commander and staff officer, again in India, in the 1930s. And Alfred Duggan’s service in the Norwegian campaign during World War II doubtless fed into his descriptions of military life in a another era.
All of these writers brought to their work a sense of authenticity: the grit and sweat and tedium of army life, the reality of combat, the sense of adventure in strange and distant lands in a time before television, the jet engine and the internet shrunk the world.
It is this sense of authenticity, I believe, that people are looking to find when they ask about ‘historical accuracy’. Not the sterile checking of facts, but the sensation of a real world, complete in all its details. Whether we achieve this by personal experience, imaginative empathy or a painstaking immersion in the minutiae of history, it should be the desired end of all our research.
Authenticity will always trump ‘historical accuracy’. Because history is changing all the time.

Ian's page is here, where this article was published as well - originally it was published here. But pay attention to Ian's two articles on the Roman Army of Diocletian and Constantine - part 1, part 2 - written by Ian.
ps
I have got Ian's two novels -  - Kindle prices are so good, and I am reading the first novel 'War at the Edge of the World.'
ps'
images from wiki Commons - Musée de l'Arles antique Arles, France, some late Roman art (IV century AD),already Christian subejct matters eg Red Sea parting in the top photo. And my favorite hunting scenes, with a 'varaz' (dzik) or wild boar hunting.
Oh, there is a French comics series titled Arelate (Arles), taking part in the Roman civitas of Arles.