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