Server
   |
Network
   |
Communication   \
   |            |
Virtual World   | These parts make the client software
   |            |
User Interface  /
   |
User


Communication

Separating the Communication from the Virtual world will introduce some 
extra latency, but will make it easier to divide the project into smaller 
parts.

Virtual World

All clients keep a local virtual world, this should make gameplay fast. 
Some simple movement prediction code would reduce lag and jerkiness.
Since the game is about big, comparatively slow things, I guess that lag 
up to 500 ms will pass nearly unnoticed (all depending on the terrain, on 
a field it won't really matter, but in a built up area it could be more 
critical.

User Interface   

The user interface lets the player give orders to the other vehicles in
the platoon, communicate with other players and drive the own vehicle in a
3D action-game environment.