Fiction that changes one or more historical events and explores the consequences, asking 'what if history had gone differently?'
Alternate history takes a real moment in time, changes one crucial detail, and follows the ripple effects. What if the South won the Civil War? What if the Nazis developed the bomb first? What if the Roman Empire never fell? The genre demands both historical knowledge (you need to know what happened to change it convincingly) and speculative imagination (you need to build a plausible alternate present from that divergence point).
Alternate history teaches you to think about cause and effect at societal scale, a skill that transfers to any worldbuilding. It also demonstrates how a single 'what if' can generate an entire novel's worth of implications. Understanding the genre helps you decide whether your historical story needs speculative elements or is better served by sticking to what happened.
The Axis powers won WWII, and occupied America lives under German and Japanese rule.
The Underground Railroad reimagined as a literal railroad beneath the Southern states.
A temporary Jewish settlement in Alaska became permanent, creating a noir detective story in an invented culture.
Pick one real historical event and change its outcome. In 500 words, describe the world 50 years later. Focus on three specific consequences: one political, one cultural, one personal (how an ordinary person's daily life is different). The best alternate histories make the changed world feel inevitable.