Alien invasion (shamelessly copied from Starcraft)
No one understood where such an odd disintegrated meteorite came from, or how it was formed. We did not understand, because we had never seen anything like it before. It was not a astronomical object at all, not made of stone or metal or ice. They were alive. These creatures survived in deep space, somehow.
The creatures were not unguided, either. We could not tell how they propelled themselves, yet as they entered the solar system they slowly altered course until they were on a direct path to Earth. They must have homed in on the strongest source of organized radiation, recognizing the Earth for the rich harbor of life that it is...was.
Their purpose was to destroy, to consume, to assimilate, to evolve. Soon after the alien vessel-creatures crash landed in South America, their spawn emerged. They covered the land, consuming and reproducing mercilessly. They replaced our great forests and grasslands with their own hideous and bizarre life-forms. Our weapons were effective enough against them, but how can you stop something that is stripping the planet bare, that regenerates from the tiniest speck left alive? By the time someone decided to annihilate the creatures with nuclear weapons, it was too late. Their spores had drifted through the air and covered the Earth. South America was a radioactive wasteland, the Earth cooled into nuclear winter, and yet still the creatures spawned from hives all over our planet. We had destroyed our planet in vain; we had lost.
There was no choice but to leave. As much of humanity as could, decamped for deep space. Some made colonies in the clouds of Venus, some on the surface of Mars, but most in the great black expanse. The Earth, our home, ceased to be to great green-blue orb we had once known. For 50 years, we humans worked to put back together the pieces of our once great civilization. Much was lost. Just as shining towers began to rise, and flowering gardens began to bloom on the formerly barren soils of Mars, the creatures struck again.
No one was paying attention to how they launched themselves into orbit. Yet here it came again, the strange cluster of alien creature-vessels, streaking across the short distance from Earth to Mars, the new center of humanity. We were disorganized, spread into factions across the solar system, and we were weak. We tried to fight, but new and terrible forms of the creatures, bred on the irradiated hellspace of Earth confronted us, and we failed again. Mars was lost.
It was decided now. This solar system that had nurtured us for so many billions of years was no longer safe, was no longer our home. We had to leave. Great generation ships were built in deep space, and launched towards all likely nearby star systems. We had lost our homes to the alien menace, but we would not lose our war. We would spread among the stars, and when the creatures came for us again, we would be ready.