In reguards to the wargaming aspect of a sim, its tough unless you can get the two waring party's and a ref to sit down and fight it out, but in the other area's its fairly simple.
Infrastructure: you need factory's to build the machines of war, here in wesworld its more a naval sim, but if you need all military branches setting it up would be as follows....
Economy
Factory's:
Airforce X ammount of factory's
Army X ammount of factory's
Navy X ammount of factory's
Civilian X ammount of factory's
"X" would depend on your nations location, but your civilian factory tally would be far higher than the rest likely combined. You would also need a GDP figure and workout how much of that GDP goes to what. You also need to find out what your nations population is in order to work out howmuch manpower you can muster for the military.
Technical
Ships and other weapons:
You would have to figure out how much materials your factory's put out and how much time it takes to build the various weapons of war, also if you want to get serious work out the time it takes to recrute the manpower for each military branch.
Working out some sort of system of keeping track of your ships "structual entegrity" and "fighting efficiency" would also be a good Idea.
This should give you enough material to work out some rules for your own sim, adjust the simplicity to your own and/or fellow simmers taste.