The Battle of Ragnar is a major battle during the Second Cyclon War, following the destruction of Admiral Nagala's flagship, the Battlestar Atlantia, Commander William Adama of the Battlestar Galactica ordered the remnants of the fleet to regroup at Ragnar Anchorage for a counterattack; however, with nearly the entire Colonial military fleet destroyed, it was a civilian convoy led by newly-sworn-in Colonial President Laura Roslin that responded to the orders.
Facing limited options, Commander Adama chose to have the Galactica defend the convoy while each ship jumped into deep space, away from the destroyed colonies. The Galactica managed to hold off two Cylon basestars as ship after ship executed faster-than-light jumps to a safe location. When the civilian ships had jumped, the Galactica then joined them. Nevertheless, less than 50,000 Humans - out of 50 billion - had survived the holocaust. Galactica and its fleet of refugees would later find that they were not the sole survivors.
At least one other battlestar, the aforementioned Pegasus, had also escaped destruction. Lt. Karl "Helo" Agathon and Lt. Kara "Starbuck" Thrace would also learn that some humans had managed to survive the Cylon attack on Caprica. While this implies that at least some humans left behind by Galactica may have survived the Cylon attack and occupation of the Twelve Colonies, it is unknown if any did.
After the fight at Ragnar, the fleet jumped past the Red Line and into uncharted space to find a new home. Unknown to the new Fleet, they ended up with thirteen Cylons within it: the Final Five, the Cavil responsible for the destruction of the Colonies and the war that followed, two Sixes, a Doral, a Leoben, a Three, a Simon and a Sharon who was a sleeper agent on Galactica. A year later, the human fleet would link up with another survivor of the attack, the Battlestar Pegasus. Eventually the survivors discovered a habitable planet they named New Caprica and were able to settle on it for a year before being invaded by the Cylons and captured. The fleet was able to escape but not without thousands of casualties and the loss of Battlestar Pegasus under command of Commander Lee Adama.
According to Saul Tigh in Battlestar Galactica: The Resistance, twenty billion people died in the attack of the Twelve Colonies; in "A Disquiet Follows My Soul", both Kara Thrace and Felix Gaeta put the number at fifty billion.
After five years of searching, the Colonials found a new world and named it Earth. Just before Earth was found, Galactica faced the Cylons in a final climactic battle and destroyed them forever. All that were left was a group of rebel Cylons that had allied with the human fleet and settled with them, while its surviving, remnant mechanical counterparts, having finally been freed by the humans and rebel Cylons to "find their own destiny", would eventually leave the galaxy in peace, never to return.
The title seems to be derived from the Norse concept of Ragnarok, the final battle of the gods.