Sidewise in Time

A Roman legion invades a quiet Missouri town, a Brontosaurus storms Atlanta, Carboniferous trees overgrow cornfields, and Viking longboats are spotted cruising up and down the Potomac. It seems time has tied the world in knots, but Minott, a bitter mathematics professor and possibly the world's greatest overlooked scientific mind, believes these are not other times, but other Earths—alternate worlds once kept separate by quantum laws, now spilling over each other with deadly chaos in their wake. As possibilities clash, Minott leads a band of reluctant students to explore and conquer the timeline-warped world.

Leinster's story laid the groundwork for alternative history and multiple universe fiction. Its influence was strong enough that the annual Sidewise Award for Alternate History was named in its honor.