Here is the full list of all 22 DEFCON achievements.
Nuke 7 billion civilians
Win an online game with at least half your nukes still remaining
Destroy a total of 50 enemy silos before they can launch any Nukes
Call a vote and convince your allies to kick a team member, a total of 10 times
Win an online game of Diplomacy
Win an online game of Survivor with at least 90% of your population still alive
Destroy a total of 50 enemy ships with Nukes
Nuke Santa and thereby end Christmas for everyone, for ever more
Lose 500 pilots due to running out of fuel
Uncover all ground bases belonging to a single enemy team before Defcon 1
Defect from an ally, then land a nuke on one of their cities within 10 seconds
Nuke Mexico, Sao Paolo and Lima in a single match
Win a 6 player game online without ever being in an alliance
Nuke Tokyo, Shanghai and Hong Kong in a single match
Launch all nukes from a fleet of six subs and escape without losing a single sub in the process
Nuke Cairo and Kinchasa in a single match
Nuke London, Rome and Berlin in a single match
Win a game online
Nuke Washington DC, New York and San Francisco in a single match
Nuke Moscow and Leningrad in a single match
Nuke a city with a population already reduced to 0.0M
Sink an entire enemy fleet of six subs, before they can launch a single nuke