The Fall of the Twelve Colonies (also known as the Cylon attack, Cylon holocaust or Second Cylon War) is a devastating, genocidal attack that renews the conflict between the humans of the Twelve Colonies of Kobol and their creations, the robotic Cylons.
The Cylons compromise Colonial defences by infiltrating their military and civilian society with new models that look human. One such Cylon manages to gain access to the Colonial Defence Mainframe and compromises the Command Navigation Program, creating countless programmer backdoors.
After the CNP is sufficiently distributed throughout the Colonial Fleet, the Cylons return from their forty-year exile and launch a devastating attack on the Colonies. The backdoors in the CNP enable the Cylons to use various electronic attacks that completely by-pass Colonial electronic defences, and inflict varying degrees of damage to Colonial military computers.
Viper squadrons experience complete power loss upon engaging the enemy, while battlestars and other support ships suffer an abnormally-high number of "equipment malfunctions" during their engagement of the enemy. This infiltration of Colonial systems, when combined with the "complete surprise" that the they achieve, enables the Cylons to sweep the Colonial Fleet aside with minimal losses; a triumph beyond their most optimistic projections.
Picon Fleet Headquarters are destroyed in the opening wave of the attack, as are 30 battlestars. In response to this, Admiral Nagala takes command of the Colonial Fleet aboard the battlestar Atlantia, and instigates a counter-attack against the enemy. Additionally, the Colonial Government orders a full stop on commercial and civilian space-travel, leaving hundreds of non-military vessels stranded in surrounding space-lanes.
After the devastating assault on Picon, Colonial President Richard Adar broadcasts an unconditional surrender to the Cylons, but his gesture is completely ignored. All senior members of the Colonial government, including Adar, are henceforth presumed killed or missing, and an emergency government contingency mechanism activates to seek out surviving government officials.
The Cylons proceeded to destroy each Colony's population and strategic centres by means of nuclear bombardment. One of the targets, Caprica City, is annihilated in a 50 megaton nuclear detonation. However, not every city on Caprica is destroyed in the attack; much of Delphi remains intact and is occupied by Cylon ground forces a short time later.
Few Colonials survive this holocaust and many that do are captured by Cylons for experimentation.