This Is How You Lose the Time War

This Is How You Lose the Time War, by Amal El-Mohtar and Max Gladstone, is a 2019 epistolary science fiction story about two enemy agents who travel through time to rewrite history for rival empires whose timelines cannot both exist. Simon & Schuster published it first, and it won the BSFA, Nebula, Locus, Hugo and Ignyte awards for shorter fiction.

Two agents are fighting a war across every version of history, one working for a technological Agency and one for a biological Garden, and both are very good at it. Red razes worlds to keep the timeline her employers prefer, and Blue prunes branches to keep hers. Then Red finds a letter left in the ash, written to her by name, and answers it.

Each message is hidden somewhere only the other will think to look, and each one is a small act of treason that gets harder to hide as the correspondence goes on. The letters move from taunting to flirtation and then to something else, and both of them know that being caught would end them. The two of them write to each other in secret while their agencies continue to fight.

It is a standalone story, and it is the joint work of two authors who wrote the agents' letters to each other.