Highly advanced civilizations have been here before us, just to be destroyed by some great global catastrophy. But for each race that has died out another has taken its place with a selected few holding on to the memories and sacred knowledge of the past race. In our vanity we think we have discovered some of the great truths of science and technology.. but we are in fact just beginning to re-discover the wisdoms and technologies past races.
In ancient Sumer, the alien Gods were known as the Anunnaki. In Greece, the Annodoti. In the Celtic lore, the Tuatha de Danaan. In the Semetic scriptures (Torah, Talmud, Old Testament, and other Apocryphal texts like the Book of Enoch), they are called The Nephilim, The Sons of God, or The Watchers.
The gods themselves had their own monarchy, with laws of succession similar to our own, and they built a global empire upon the Earth, with great cities, temples and monuments, and mighty nations established on several continents.
They created mankind as a slave race