I think just throwing it on a budget slider would be the optimal method; but with basic support issues. Have it so you delegate it in the usual method (Eg. Science at say, 40% of taxes, Entertainment at 10% and the Military at 30%) in the Domestic window.
Basically, all governments would have a basic support limit for thier Army free, just so you can get some defence without spending megabucks. Then, going to Domestic, you can adjust the slider, from 0-100 % of taxes taken. (Affecting Science and Entertainment as thus.) Except each percentile equals a certain amount of units. So, say, 1 % = 10 units, then 30 % = 300 units. But have it so you can get it in increments of 1 %. Also, going over budget should take away from your other domestic areas. So, having 310 units means that five percent will be taken away from Science or Entertainment. (The computer chooses. Should've stayed in budget.
)
And after that, when you declare war, there should be a certain part on the military slider that you can "drag out" for unit support during wartime. So if your troops aren't on your own soil, then you have to drag out a percent for the support limit according to how many troops you're sending into combat in another nation's borders or overseas anywhere (border or not). It'd look like this:
Science:
20 %
|--------|------------|
Entertainment:
10 %
|----|----------------|
Military:
30 % + 10 % logistical support.
Basic Military Cost Logistical Cost
|[--------30%--------]{--10%--}------|
Have it so whenever you're in wartime, the logistical value is activated. Each percent of logistics means you can send ten units into enemy territory without suffering "damage" from attrition. Fighting on your own soil is free; upon moving them out of your territory, they cost money in logistics. (Applies to ships as well; at all times.)
Trying to send out more than your logistic support results in severe attrition damage.
Therefore, if you want wars, it'll cost you some serious cash.
A system like that would be good I think...
