During the First Cylon War, Galactica along with the Columbia and one other unnamed Battlestar tooks part in Operation Raptor Talon; where the Columbia is destroyed with all hand.
Many years after the war, Galactica was one of the oldest ships in the fleet, and was scheduled to be decommissioned and turned into a museum. Various conversions were taking place and most of her onboard squadrons were removed.
During the decommissioning ceremony, the cylon attacks on the colonies began. After a battle, including many direct strikes, Galactica successfully jumped away to the Ragnar Anchorage depot.
Galactica then leads and defends a small fleet of civilian ships on the search for resources and a new place to settle.