This game is a simplified version of a game called GalCon (itself a take-off on Galactic Conquest) that was the also the subject of the "Planetwars" Google AI Challenge from the fall of 2010. There are likely some good resources from the winner of that competition.

In short, a good approach is using a form of utility theory based on:

• The number of units at a given target
  
• The number of your units already enroute to that target
  
• The number of enemy units already enroute to that target
  
• The time it will take to get to the target
  
• The value of taking the target (if there are differences in production rates, etc.)
  

By using those inputs, you can calculate the feasibility of taking that target (risk) compared to the value of taking it (reward). At that point, game theory kicks in (a la Rock-Paper-Scissors and Prisoner's Dilemma) so you can determine what good strategies are to follow. For example, should I will take many smaller targets or battle the other player for the big target in a war of attrition? Often times, there is no right answer to that but rather it needs to be a combination of reactive to the other player's moves and bold at making proactive decisions.

If you need help, I can likely code up the AI for you fairly quickly based on the work I did for that competition. My bot was successful as long as it didn't bail on a specific (untracable) error on the very last move before I would win. (The challenge framework is a nightmare to deal with.) Please note that I do AI consulting for a living so this wouldn't be free. However, it would be pretty straight forward and would pay off in spades for your game. (Which looks incredibly fun!) Ping me if you need help.