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