Battle of Cimtar
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After 1000 yahrens of war, the humans of the Twelve Colonies vie for peace with Lord Baltar acting as diplomat. The peace is to become official at Cimtar, a moon believed to be neutral territory. Instead of peace, the Cylons launch their largest onslaught ever against the Colonials. The Cylons use a two-pronged attack, striking at both the peace delegation (including the battlestars) as well as the Twelve Colonies.
Cylon fighter groups of Cylon Raiders (numbering a thousand total, along with support fuel tankers) hide near the Cimtar moon in wait for their chance at attacking the battlestar fleet. The basestars are left free for bombardment of the Colonies.
Commander Adama of battlestar Galactica receives reports that a Viper patrol has discovered the massive, hidden Cylon Raider fleet near Cimtar. Adama warns President Adar, President of the Colonies and commander of battlestar Atlantia of an imminent attack, but Lord Baltar sways Adar from ordering a defensive posture for the battlestar fleet.
Adama readies Galactica's defenses against orders. The battlestar is able to defend herself when the Cylons arrive, but the other battlestars, following President Adar's pacifist lead, are not so fortunate and can launch only a fraction of its fighters in time.
The Cylon fighters use a variety of tactics, including kamikaze methods to destroy the battlestar fleet. The Cylons destroy battlestars Atlantia (with President Adar aboard), Triton, Pacifica, and Acropolis.
Leaving her Viper pilots behind temporarily after getting reports of bombardment of the Colonies, Galactica heads towards Caprica in a vain effort to protect her homeworld.
During Galactica's flight to the Colonies, the battlestar received reports as the Cylons cleared the jamming to allow their own targeting and communication equipment to function. In one broadcast, 31 cities were "known to be under heavy attack" on an unknown colony while another report mentioned a second wave coming in.
Cylon basestars sneak by Colonial defenses, allowing an undefended attack on the Colonies. The home planets are bombed and the Cylons exterminate almost all of the human population of the Colonies. Caprica's destruction is assured through the efforts of Karibdis, one of Baltar's agents, who disables Caprica's defense systems and is held responsible for a million Caprican deaths (Murder on the Rising Star).
A handful of humans are tasked by Adama, now commanding the last known surviving battlestar, to flee the Colonies in whatever spaceworthy ships they can find to create a motley caravan that abandons Colonial space, initially for the nearby planet of Carillon where they obtain supplies for the long journey to Earth. Their exodus -- via the dangerous but poorly defended Nova of Madagon rather than the logical waypoint of Borallus -- takes the Cylons by surprise, who change their tactics to eliminate an now-elusive, moving target.
Lord Baltar is initially considered expendable by the Imperious Leader after the human's traitorous work and set up for public execution. However, the Leader's sucessor reconsiders Baltar's execution and provides him with the means to chase Galactica and her fleet to finish humanity once and for all.