Draw up a map.  It can be a simple map.

The map should resemble many many loose states.

Basically I want to write something like a romance of the three kingdoms playthrough.

There should be named generals from each land.  They should have varying strengths and weaknesses.

As should the kingdoms themselves.

There should be a small blurb about every general I can reference when writing.  This could be a series of 
short stories. (think animorphs length)

perhaps go in anime style

perhaps literally roll the dice.  Give each general tactics and abilities.
give them stats like warhammer (simplified)

give the abilites (like trample manuvers, hammer and anvil, research real combat tactics.)

Which everside wins the die roll, which will be padded by determined by army size, skill, and specific manuver used, will be writen about as the victor of that skirmish

Hell, it could be one book per battle. Or perhaps, one book per turn.  Decide how every country is or isn't going to invade on turn one, write about each country in depth for the first book.
Perhaps start with an invasion.

Chapters can be named for the general whose point of view is being taken.

Alliances, joint attacks... etc...