At a more advanced level, larger games get interesting again thanks to Alliances. These can be formed or broken at any time, but the security they offer is soured by the fact that your fair-weather friend gets to see the position of all your units (except, cleverly, subs). You don't have to betray your alliance to win, but you can bet that any teammate with a lower score than you will.
The pruned unit selection, simple relationships and fixed playing field give DEFCON more in common with chess than most RTS games. It's not a game to get lost in for hours on end, but it is one you'll get the urge to play again and again for months to come. The fact that it's only 15 bucks seals the deal. Buy it immediately.