You got my interests even further...
Also, there needs to be a scoring system for the chance of success or failure in combat, damage on one type against another, critical damage, difficulties of one type against another (i.e. like what you said with infantry against tanks), distance, range & accuracy. Like if our Game Master should use a dice or some other random scoring piece to do all of that.
As for the 5 unit categories, should there also be sub-categories, since there could be different types of each unit? Infantry, for example, could come as Riflemen & Anti-Tank (with better chances against Armor), and Armor or Navy could come with units that specialize against Aircraft.
Oh. and don't forget terrain and weather. Those are big deciding factors in strategic warfare.
All that aside for now, I want the Athenian faction, and I could find ways to make them from ancient Greece to 2100's era... 
Okay, thanks for giving me more information, and I'll integrate those, so here are the sub-classes
Infantry: Rifleman, Officer, Anti-Tank soldier, Machine Gunner
Tank: Light Tank, Medium Tank, Heavy Tank, Flame-thrower
Aircraft: Fighter, Bomber, Naval, Helicopter
Navy: Frigate, Destroyer, Cruiser, Battleship, Aircraft Carrier
Artillery: Mortar, Heavy Artillery, Multi Rocket Artillery(shoots many rockets at once)
And each player is in control of a single nation, and that is for all the games unless you want to change.
Weather is determined by season.
Summer is hot, with little rain, so is good for invasion, but not campaigns, Spring is rainy, and temperate, so its good only for certain battles, Fall is temperate, dry, so best time for campaigns, Winter is cold, and so is not good for any type of warfare.
The percentage of who wins must be considered by a moderator, or game-master, who takes into account the number of tanks, troops, artillery, weather, and terrain to decide who wins, he must be unbiased!
There are also types of battles that can be chosen, trench warfare, guerilla, and maps. But its not just a war-game, you can form alliances with other players, or trade routes.
Tell me how I can improve this.
But here's a list of what we can do:
WARFAREDeclaration of war, you can declare war on other players, although it's recommended that you prepare for war.
Planning, form a plan for how to subdue the enemy.
DIPLOMACYAsk for a trade route
Send an embassy in
Sign a non-aggression pact
Form an alliance
ColonizationColonize unclaimed territories
Take over enemy territories
MAPThis is something that is added everytime a major event occurs, the main map is on the very top of the game, and changes when someone does something important. Ex: America sets up a colony in North Africa, or, Germany declares war on Sparta. When wars occur, the two sides with all participating belligerents in two colors, to show the different size.
LEADERSLeaders are the players in control of the nation their playing as, and incharge of their country's ingame affairs.
United States(Chomper98)
Great Britain
France
Spain
Germany
Japan
China
Confederate States
Russia
Athens(Mirumoto Kenjiro)
Sparta
Italy
Assyria
Babylonia
Egypt
Netherlands
Just let me know what country you want to be. This role-play will start as soon as we find a moderator or game-master. I will also make a map of this.
Sorry for the badly drawn map, I don't have a scanner, and I will try to make a better map for the game later. But here it is right now.