The craft of building a world where real history took a different turn, then following the ripple effects with honesty and rigor.
Alternate history worldbuilding starts with a simple question: what if one thing in real history had gone differently? What if the Roman Empire never fell? What if the printing press was invented in China first? The craft is in following that change honestly, tracing how one divergence reshapes politics, technology, culture, and daily life across decades or centuries. It's not just changing one event; it's understanding how that event was connected to everything else and letting the dominoes fall.
Alternate history forces you to think about causation in a way that pure secondary-world fantasy doesn't. Because your readers know the real history, every change you make is measured against something concrete. That tension between the familiar and the altered is where the genre's power lives. It also sharpens your worldbuilding instincts generally, because you learn to ask 'why did things turn out this way?' before asking 'what if they hadn't?'
The foundational alternate history novel, exploring a world where the Axis won WWII and America is divided between German and Japanese occupation.
Clarke blends alternate history with fantasy, introducing English magic into the Napoleonic Wars with meticulous historical texture.
Trace the ripple effects. If Rome never fell, medieval Europe looks completely different: different languages, different religions, different technology. Follow the chain.
Historical figures were products of their contexts. If you change the context significantly, those people might never have been born or might have become very different. Be thoughtful about it.
Do your homework. You need to understand why things happened the way they did before you can plausibly argue they could have happened differently.
Pick one real historical event and change its outcome (a battle lost instead of won, an invention that failed instead of succeeded, a leader who survived instead of dying). Spend 15 minutes writing a timeline of consequences: what changes in the next year, the next decade, and the next century. Then write a short scene set in that altered world, showing one ordinary person living with the consequences.