Lantean-Wraith war

The Ancient-Wraith War was the century long conflict which lead to the destruction of the Ancients as a civilization.

After travelling to the Pegasus galaxy in Atlantis, the Ancients, now known as the Atlanteans, seeded many planets in Pegasus with Humans, while colonizing others. One of these planets, however, was home to a species called the Iratus Bug, a creature possessing considerable immunological capabilities. The bugs began to feed on the humans seeded there, and as time went on, they began to mutate, taking on many advantageous characteristics. This brought about the creation of a hybrid species known as the Wraith. Eventually, the Wraith, needing to feed on Humans, began a conquest of the Pegasus galaxy.

For one hundred years, the Ancients and the Wraith warred with each other. While Atlantean technology was far superior to the Wraith's organic technology, the sheer numbers of Wraith soon began to overwhelm them. Desparate, the Ancients began to take more drastic measures, such as Project Arcturus, an attempt to extract Zero Point Energy from their universe, and created the Asurans, nanites which were intended to destroy the Wraith on a cellular level. Both failed, with disaterous conaequences.

Eventually, all that remained was Atlantis, which was submerged in the ocean to protect it from the Wraith. The Ancient Council sent a delegation protected by their most powerful warships to negotiate a truce, but the Wraith destroyed them. Having lost the war, the Ancients left Atlantis through the Stargate and returned to Earth, to live out the remainder of their lives.