Air: Remote-controlled pilotless drones. They'd primarily assault enemy ground settlements, and wouldn't really be equipped for drone/drone combat.
Sea: Entirely missile/missile, like playing Battleship. No reason to come any closer when you can wreck the enemy from afar. Possibly underwater submarine drones for closer combat.
Ground: Cyber attacks, long-range infrastructure damage wouldn't require armies. Any armies would be used for threatening, and if needed would probably win by marching in after the enemy has been sufficiently bombed to hell.
Cyberspace: Now that's a real question. Study that.