Visitors from the Alpha Centauri star system

Five years ago on New Year's Day of all days the Earth suddenly had an alien fleet in orbit around it. As we would learn days later when they cut in on every television and radio broadcast on the planet these Aliens' called themselves the Centauri after their home star we call Alpha Centauri. The told us the had crossed space in order to help other world avoid the mistakes of their past. At first they were very helpful but after about 6 months things changed for the worse.