Baxter, Stephen. "Mars Abides"
What if: The Mariner 4 probe revealed that not only did Mars have a breathable atmosphere but that there was vegetation on the surface.
Summary: An alternative Cold War and space race, with the first human landing on Mars in July 1976 but culminating in the One-Day War of 1983.
Published: In Obelisk, Gollancz 2016 (147321274XBUY, 1473212766BUY); and The Year's Best Science Fiction: Thirty-Fourth Annual Collection (ed. Gardner Dozois), St. Martin's Griffin 2017 (1250119235BUY, 1250119243BUY).