Invasive Species. Invasive Species, #1

An asteroid brought them. We built giant robots to keep them from terraforming the Earth! Earth is under attack. Not from an enemy with guns, tanks, fighters or anything else we spent centuries training for. Fifteen years ago Earth was almost destroyed by a planet-killer asteroid. In a last ditch effort we threw nukes at it. We succeeded in breaking it up, but not enough. Five large fragments crashed down to Earth.
We thought surviving the dust clouds and destruction of planetary infrastructure was the hard part. We were wrong. Humanity wasn't prepared for what the asteroids brought with them from the stars: The seeds of our destruction.