Wow!  Lots of comments!   

   I am familiar with 'The Art of War' but its one of those books I have never got around to reading.  Perhaps now I will (tisk, and I call myself a wargamer eh?).
All of these comments and links are very useful and so thanks for them.  I think I need to be more clear though, I'm not actually drawing modern warfare tactics and employing them with ancient armies, I am merely drawing a comparison as modern armies as these are composed of divisions and are in many ways committed to battle seperately.  Ancient armies are clearly committed as a whole and not Battle Group by Battle Group to use FOG expressions.  
I was just wanting to get clarity on how these wars were fought.  It seems that they happen by simply raising another army providing the commander survives, and they have enough time and / or money to do so.  
I will be sure to look heavily into logistics.  Basically, I will use cities that provide an amount of equipment, food and money to whomever controls them.  Each troop type will have different upkeep costs for this.  If they run out of supplies, then bad things happen.  I also have a system for trying to send messages which can be waylaid or other things happen to them.  
Does anyone think this is a good idea or can you think of reasons why this would be bad?