Anubis used an alien device, perhaps originally built by the Ancients, to destroy Earth's stargate. By establishing a wormhole, minute amounts of energy were transmitted through the stargate causing a .1% rise in power that was retained by the gate's internal capacitors. The stargate is a giant superconductor capable of absorbing huge amounts of energy, however the gradual energy buildup eventually would exceed its capacity. A potential explosion of the charged naquadah in a blast of 2000 to 3000 megatons threatened to obliterate Colorado, with resulting environmental effects enough to destroy all life on Earth. The destruction of the planet would appear to be a natural disaster and therefore beyond the terms of the Protected Planets Treaty.
Unable to prevent the buildup of energy, even through a closed iris, Dr. McKay theorized that an electromagnetic pulse sent back through the incoming wormhole would destroy the weapon, but his plan was unsuccessful. A second plan was devised in which the stargate was attached to an X-302 and transported through a hyperspace window before it reached the point of detonation. The mission was successful and Earth was spared, but was left with a single stargate. Teal'c, Bra'tac, and Rya'c were able to locate Anubis's weapon, and it was destroyed by Rya'c in an aerial assault from a death glider.
Cross Reference: Ancients, Anubis, Beta Stargate, Bra'tac, Electromagnetic Pulse Generator, Rodney McKay, Larry Murphy, Protected Planets Treaty, Rya'c, Stargate, X-302
Episode Reference: Redemption