One hundred years before Ender's Game, the aliens arrived on Earth with fire and death. Earth Afire by Orson Scott Card and Aaron Johnston is the story of the First Formic War.
Victor Delgado beat the alien ship to Earth, but just barely. Not soon enough to convince skeptical governments that there was a threat. They didn't believe that until space stations and ships and colonies went up in sudden flame.
And when that happened,

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Traces the events of the First Formic War a century before the events of Ender's Game, following the unsuccessful attempt of Victor Delgado to warn skeptical Earth governments of an imminent alien invasion and the efforts of Mazer Rackham and the Mobile Operations Police to meet unprecedented threats.

A hundred years before Ender's Game, humans thought they were alone in the galaxy. Humanity was slowly making their way out from Earth to the planets and asteroids of the Solar System, exploring and mining and founding colonies.
The mining ship El Cavador is far out from Earth, in the deeps of the Kuiper Belt, beyond Pluto. Other mining ships, and the families that live on them, are few and far between this far out. So when El

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The mining ship El Cavador, beyond Pluto, detects a fast-moving incoming object headed toward Earth. The crew decides it's probably not important, but they're wrong: it represents the opening wave of the first Formic War.