Historical fiction attempts to present life in eras before living memory.
Some consider histfi a setting rather than genre since it hybrids so often (but fantasy isn’t so considered when it crosses with romance, mystery, etc.). I think that’s right, and it kind of is about fantasy or science fiction. Like fy/sf, historical fiction uses a great many templates to structure its stories. What all histfi has in common is the setting before living memory. When you get into Vintage fiction (pardon me for coining a term), you are in the living memory of some, but a 20-year-old author would have to research their parents’ youth in the same way as they would something a century ago. It just markets differently, and the genre tool is all about figuring how to improve marketability in revision.
The combinant forms are not all from the last twenty years, though historical mystery gained greater popularity. John Dickson Carr wrote them in the 1950s (Captain Cut-Throat). Today, from 10,000 BC to the 1960s, they’re popular because modern evidence detection makes amateur detectives more difficult to write.
Historical romances range from sweet to erotic, but they run on the romance template. They also date back to Sir Walter Scott in the late 1700s.
Spy stories, thrillers, military fiction, inspirational fiction — many stories can increase audience by hybridization with the historical setting. People like a chance to leave today for some place different, if not simpler.
Is there “straight” historical fiction? Marketing-wise, yes, but it can be described as a historical domestic drama (Norah Lofts’ The Maude Reed Tale) but, if it has battles and escapes, as historical action/adventure (almost anything by Rafael Sabatini). Very large multi-character histfi is historical saga (War and Peace), as would be a multi-generational one (Roots).
But there is a histfi semi-template of its own. In fact, two.
The first has as protagonist the secondary or little-known but actual historical character, but occasionally a major player. If fascinated by one already, this is your plan. Their life provides storyline, but you decide theme and plot, and fill in unknown parts of their life to create a full character, more or less of your own invention (the d’Artagnan of Dumas, pere, was one). This sometimes partakes of the Secret History, straight. (As opposed to Secret History, science fiction or Secret History, fantasy. See below.)
Usually the protagonist is completely invented who witnesses history, meets actual known people, and resolves historical mysteries or simply the problems of a life back then. The invented character may just live a life of the time, with other invented characters (very common in historical romances) and never meet famous persons or attend famous events. The latter used to be oddball, but is now the most common.
That last is normal in frontier fiction, whether settling Australia, Canadian voyageurs, or the classic Western, which is defined by publishers as a rural historical in North America set west of the Mississippi Valley between 1865 and 1900. These historical outdoor adventures emphasize wide open spaces, like stories of explorers or wanderers, or the work of settlers.
For prehistoric fiction many will think Jean Auel, but Jack London wrote Before Adam. This small but fascinating sub-genre tends to specialist authors because they require background reading more difficult to find than historical eras. These include stories of American tribes before the Europeans, with identical tribal situations, technology, and settlement/migration questions. Some writers have been bothered by a histfi sub-genre that’s pre-historic, but need to loosen up. It’s not about what’s written in ancient scrolls and old books any more: it’s about what we know and otherwise fill in about times before ours.
Aubrey & Maturin or the older Horatio Hornblower series exemplify historical naval fiction or historical sea stories.
Alternative history or alternate history can have its own criteria. It is all speculative in that you choose a divergence in history and follow out the changes that would make. But if it is “pure” — Here’s the world if Napoleon hadn’t attacked Russia and the Bonaparte dynasty still ruled — it is really done exactly like historical fiction, except you have the hair-tearing fun of working forward through arguable alternatives. The presentation is exactly the same. Stop and think of the average person on the street who thinks movies like Titanic or Gladiator or El Cid are historically accurate films. They cannot tell pure althist from regular histfi unless you hit them with something big like “Hitler won”: they don’t know enough history or historical anthropology. So the templates and presentation of world-building are exactly the same.
Speculative althist — with time-travelers actively involved, or the addition of other specfi elements like magic or ETs — runs on the specfi templates.
If the idea of combining all the work of histfi research with the world-building of, say, an impinging magical world or alien landings, made you dizzy … now imagine you present it as a mystery. (Screaming is allowed.) Philip Jose Farmer did this in The Other Log of Phileas Fogg, a science fiction Secret History.
But before you get too settled on the idea that what you have is historical fiction, I strongly suggest you consider just how much work of what kind you want to do. Your story may actually be better off becoming ahistorical fiction in your revision.