Bridge Over Troubled Water

I've set up a very simple mechanism for war between the arabs: when at war, the two nations battling have one chance to make a hit every turn. If one nations scores five times and has an advantage of two scores over the opponent it wins the war. Another case is that when a nations has an advantage of three scores over the adversary: in that case it wins without the need to have the minimum of five scores. The chance to score a hit is based upon the usual proficiency value.
I think this system, despite simple, models war between arab countries in a good way.
I'm now working on how a war between arabs affects the number of troops that the same arab countries can deploy against Israel. I thought this system, let's take for example Syria: it has -let's say- 120 units, which is 100% of its strength deployable against Israel. If Syria comes to war against Iraq a floating percentual value of its army will not be deployable anymore. If it comes to war against Jordan or Lebanon there's another percentual value subtracted from the total. This way if the player can establish an alliance with Iraq against Syria, he will have to fight against only a fraction of the total number of units that Syria has.
The problem is to decide how this floating percentual value actually floats... it could be that it floats based upon the relative proficiencies of the armies involved (if Iraq has a more powerful army Syria will be deprived of more units than it was if Iraq had a terrible army).