Fall of the Twelve Colonies

The devastating openning wave of the Second Cylon War, in which the Cylons used a programming backdoor to infiltrate and shut down Colonial Fleet systems. Almost all ships that tried to engage the forces of the Cylon surprise attack suffered immediate and total power failure, "like someone turned off a switch". As a result the Cylon fleet suffered almost no loses and effortlessly destroyed most opposition.

The Battlestar Galactica, helmed by Commander Adama, was the only Colonial ship unaffected by the Cylon's computer infiltration, because it's computer systems were never networked. As such, it was presumably the only ship that ever mounted any real defense against attacking Cylon ships. Though the Galactica lost all 20 of her Viper Mark VII's in their first sortie against the Cylons because theier networked computers were vulnerable to infiltration, she also had 40 Viper Mark II's board as part of it's recent decomissioning ceremony. The Viper Mark II's were an old but robust to begin with design, and most importantly they had never had networked computers installed and were therefore also unaffected by the Cylon computer virus. The Galactica was able to re-arm and deploy its Viper Mark II's and repel several attacking squadrons of Cylon Raiders, even surviving a direct hit from a Cylon tactical nuclear warhead.

The Cylons proceeded to start nuking each of the Twelve Colonies, including Caprica. After Picon was nuked, the Colonials broadcast an unconditional surrender, which was completely ignored by the Cylons.

Picon Fleet Headquarters was nuked in the openning wave of the attack, as were 30 Battlestars, "a quarter of the Fleet". Admiral Nagala subsequently took command of the fleet aboard the Battlestar Atlantia. Soon after, the Atlantia was lost with all hands, and Commander Adama aboard the Galactica broadcast a message that he was taking command of whatever was left of the Fleet, and ordering a retreat to a rendez-vous point at Ragnar Anchorage.