Generally, EMERGCONs are a state of readiness following an ICBM (intercontinental ballistic missile) attack. Other forces go to DEFCON 1 during EMERGCONs. There are two types of EMERGCONs:
Defense Emergency: A major hostile attack on United States and/or allied forces overseas, and/or an overt action made against the United States. This must be confirmed by a unified commander or higher authority.
Air Defense Emergency: A major hostile attack by aircraft or missiles seems probable, is imminent, or is taking place on the continental United States, Canada, or Greenland. This declaration is made by the Commander in Chief of NORAD (North American Aerospace Defense Command).
Although the United States military has gone into defense readiness various times (such as the Cuban Missile Crisis), it has never had a major EMERGCON (the somewhat regularly occurring suicide bombings in the Middle East were probably not considered major attacks on allied forces), except possibly during the September 11, 2001 Terrorist Attacks on the World Trade Center and The Pentagon.