There are three tiers of structures in the US faction.
Defcon 3 - Basic buildings and units
Defcon 2 - Advanced buildings and units
Defcon 1 - Very advanced buildings and units
- You already have access to the Defcon 3 tab. To get access to Defcon 2 buildings you have to build a Refinery which is in the Defcon 3 tab.
- Once the Refinery is up you can learn Defcon 2.
- Once Defcon 2 is learnt you can learn Defcon 1