In any given bookstore or library you are likely to find historical fiction shelved in with the fantasy and science fiction: e.g., Harold Lamb ends up near Fritz Leiber. This is an understandable mistake. There is a certain amount of overlap involved in the content.
Consider historical fiction. Ostensibly the authors are telling stories based on actual events, fictionalizing historical occurrences. But even with the most well-documented eras of history not every incident can be certain. Inevitably the author must speculate. The more remote the era, the less well-documented, the more shrouded in myth, the greater the degree of speculation. This lends itself to legend making, to dabbling in the areas commonly the province of fantasists. Thus you see Bernard Cornwell and Mary Renault shelved next to Glen Cook and Mike Resnick.
Fun fact: I’m currently reading Bernard Cornwell’s latest, “The Pagan Lord” and the cover blurb is an accolade from George R.R. Martin.
Tales of King Arthur are prime real estate for this juxtaposition of genre borders. Given the likely ahistorical nature of the main characters and the accreted layers of myth, anything goes. Booksellers can be pardoned for shelving T.H. White, Rosemary Sutcliff, Mary Stewart, and Parke Godwin (to name but a few toilers in the Arthurian gold mines) in fantasy, despite whether the intent of the novel was to present meticulous historical research, to set a sword-and-sorcery yarn within a familiar background, or to just have a lark.
The connection cuts the other way as well. Speculative fiction authors make use of history for plot, inspiration, or setting. Harry Turtledove wrote an entire series retelling World War Two in a fantasy world complete with dragons and magic. Martin’s “Game of Thrones” owes a debt to The War of the Roses. The list continues to grow.
So I may get some use from that History degree after all.