The First Siege of Atlantis was the final and most significant battle in the century-long Lantean-Wraith war that took place 10,000 years ago.
Due to the Lanteans' own overconfidence, the Wraith were able to acquire several Zero Point Modules and used them to power a cloning facility to bolster their numbers faster than they could be diminished. With the much needed advantage they sought, the Wraith were able to absorb their losses and meet the Lanteans head-on time after time. Quickly absorbing Lantean territory into their own, the Wraith steadily advanced toward the Lantean system, destroying every Lantean settlement they could find in the process until there was little standing between them and Atlantis. (SGA: "Rising", "Spoils of War")
As the organic fleet exited hyperspace, they met quick destruction from afar via the Lagrangian Point satellites located throughout Lantea's system. Despite holding off the Wraith temporarily, the satellites quickly found themselves overwhelmed by sheer numbers. The Wraith continued to pour their fleet into Lantea's system, and using kamikaze charges and overwhelming firepower destroyed the majority of satellites in their path. (SGA: "The Defiant One")
Now undeterred the Wraith stationed their ships in Lantea's orbit, and directly into Atlantis' weapons range. The City-ship’s Drone weapons quickly destroyed the besieging fleet (with the exception of one Wraith cruiser, which crashed into the planet's ocean and sunk to the bottom), but the Wraith would not be swayed from their goal. Determined to destroy their weakened enemy regardless of the cost to themselves, the Wraith continued to send their quickly diminishing fleet into Atlantis' orbit. Despite managing to destroy the attacking fleet every time, Atlantis' shield was under continuous bombardment, its drone supply was being used up faster than it could be replaced, and with little to no remaining resources even the power supply would go dry eventually. (SGA: "Submersion")Seeing no way of winning the war yet still capable of dealing tremendous damage, the Lanteans sent a delegation under the protection from some of their most powerful warships still remaining from the long years of war in the hopes of negotiating a truce. It was never to be, however, for the aggressive Wraith were empowered by the knowledge that they were on the edge of winning the war and making the Pegasus galaxy their own. The Wraith attacked the delegation, destroying both it and the Lanteans' only means of effectively continuing the conflict. The Lantean-Wraith war had been lost. (SGA: "Before I Sleep")
- "In an effort to save the last of our kind we submerged our great city into the ocean. The Atlantis Stargate was the one and only link back to Earth from this galaxy, and those who remained used it to return to that world that was once home. There the last survivors of Atlantis lived out the remainder of their lives."
It is possible that they had hoped to return with reinforcements from their brethren that might still be alive in the Milky Way, or possibly enlist the help of the humans from Earth, whom they left behind, to help them rebuild their civilization. Whatever the case, upon returning to Earth all they found were fragmented tribes of spear-wielding hunters living in an environment too harsh for their civilization to return to its former glory. Disillusioned, the Lanteans decided to lay the seeds for which the future descendants might use: some interbred with man in order to pass on the Ancient Technology Activation gene and helped create the earliest civilizations, some sought ascension, and others went on to forge the Alliance of Four Great Races. (SG1: "The Pegasus Project")
- Though the exact order of the events described throughout the series is unclear in regard to the First Siege, it was likely deliberate to give the writers enough freedom to add new information as they see fit.
- Like the Alteran division and the Ancient contagion that swept through the Milky Way, the First Siege of Atlantis (even the entire war) was a significant event that leads to the current state of the known universe.