This is Where I Came In

Imperium, Baen, 2005 was a collection of three short novels by Keith Laumer first published in the 1960s. The collection was edited by Eric Flint with an preface by Harry Turtledove titled "This is Where I Came In". The premiss behind Laumer's stories is that there is a multiverse where different strands of history co-exist based on historic events going in both (or more) directions at once. Turtledove later used the concept in his Crosstime Traffic stories.

In the preface, Turtledove explains how he read the first two stories when they came out in the 1960s and how they, along with Lest Darkness Fall by L. Sprague de Camp and The Man in the High Castle by Philip K. Dick, introduced him to the Alternate History sub-genre. He thinks that H. Beam Piper first showed a vehicle that traveled between timelines in his Paratime stories but felt that Laumer's were the Model-T of the genre.