3 ideas
In a world where Napoleon won at Waterloo, three men a century later build a machine to reach back into the battle so that he loses, and the intervention succeeds through Lisa's cry. The correction erases Hervey, while Dieujeu survives to tell the story.
The novel sets its baseline in a world where Napoleon won and treats our real history as the speculative alternative its characters try to build. This inversion makes the familiar outcome of Waterloo look contingent and chosen rather than inevitable, so the reader sees their own present as one branch among possible ones.
The novel's premise has Waterloo balanced finely enough that changing Captain Hervey's conduct reverses its outcome. Yet the ending stresses how little the wider world changes as a result, so the book shows the battle turning on a narrow contingency without claiming later centuries turn with it.