The twelve colonies of Kobol: Aquarion, Aerilon, Canceron, Caprica, Gemenon, Leonis, Libran, Picon, Sagittaron, Scorpia, Tauron and Virgon built robot servants called Cylons. Curious about the legend of Earth they sent a spaceprobe through the galaxy looking for the 13th colony. The spaceprobe was operated by cylons, who in those days were loyal to humanity. Eventually the spaceprobe crashed on Earth at Roswell, New Mexico in 1947 where the cylons onboard were seen as a threat from outer space. Fearing invasion, American scientists reverse engineeered the cylons and re-programmed them to fly a return mission to the twelve colonies and rebel against their masters.
Thus, we of Earth are responsible for the entire human-cylon war.
This is not canon. It's the story I made up while I'm waiting to see how the final episodes turn out. I always do this. Make up my own versions of stories while I'm waiting to see what really happens.