The Twelve Tribes of humanity,(named after the Zodiac), had been forced to leave the world of the gods, and founded the Twelve Colonies. Eventually they had progressed far enough scientifically to produce artificial intelligence. The robots, known as Cylons, served humanity for a while happily, but they began to yern for freedom and equality. When the human masters wouldn't give it the Cylons decided that they would take it by force.
What followed was the first Cylon war. Eventually after enormous casualties on both sides an armistace was reached, and the Cylons disappeared into the darkness of space. Although a station was set up for peacefull communication and debate between the humans and the Cylons there had been no communication, or indication as to what the Cylons were up to for 40 years. Humanity prospered and the Twelve Colonies flurished. Then in an instant everything changed. They Cylons destroyed the monitoring station, and moved into Colonial space.
The Battlestar Galactica was being retired after 40 some years of service. Its commander William Adama would recieve a new commision. However before the ceromony could be compleated the Cylons attacked. The ships surrounding Caprica, the location of the Galactica, attempted to fight back but the Cylons transmitted a virus that crippled them with power failure. Within minutes the planet was defensless except for Galactica.
However the numbers were too great for one ship Adama, and the crew fled the system with as many civilian ships as possible. They later learned that the rest of the Colonies had suffered the same fate, and that they were all that remained of humanity. With the Galactica as their protector the "fleet" set out under newly appointed President Laura Roslin to find a new world to call home. Then after the president has a vision the "fleet" sets out to find the last, or "lost", colony of humanity. aka: Earth