DEFCON, short for DEFense readiness CONditions, outlines progressive alert postures used primarily by the Joint Chiefs of Staff. Describing phased increases in combat readiness, these are:
Historically, the US has rarely declared a readiness level beyond DEFCON 3, however the SGC reached DEFCON 2 during the threat of invasion by the Replicators, and the Entity. As Anubis's fleet launched an attack against Earth, President Hayes ordered the US to DEFCON 1.
The following year, when Goa'uld symbiotes infiltrated the highest levels within the Russian government, both the US and Russia, as well as China, progressively heightened their levels of military preparedness. At DEFCON 2, the Air Force Space Command put all of its missile wings on alert, the first B2 left Whiteman, FL SATCOM sent out an emergency action message, nine missile submarines were at sea and the rest prepared for immediate sortie, and the President and his team left for Andrews where the National Airborne Command Post awaited their arrival. The Russians reached a state of full military preparedness, and the US responded by going to DEFCON 1 before the crisis was averted in the final moments before an attack could be launched.
Cross Reference: Anubis, Entity, Replicators, Russians
Episode Reference: Nemesis, Entity, Lost City, Full Alert