Maybe use GURPS Mass Combat Rules

I was actually strongly considering talking about this at some point; I have a copy of the PDF on my computer, and these rules are quite excellent at

• Allowing you meaningful choices for strategy in battle
• Quickly resolving fights
• Keeping track of losses and overall force strength
• Allowing individuals to shine through (if that's important - like a showdown between Rommel and Patton where their own uniqueness can have some effect on the battle)

I will break it down for everyone in another post when I get home. I ran a mock battle with some friends and really enjoyed it! It was a Technology Level (TL) 3 force, which is roughly middle ages to Renaissance. We would likely be in TL 6 for the early part and then into TL 7 for the later part, if we went with @PMYA's historical option.

Modern would be TL 8.

EDIT: This is from 1993, which I believe was in 3rd edition. They published a PDF with mass combat rules for any time period in 4th edition, which is what I have. While I can't publish the whole book (although sharing it may be an option cough cough), I can run down the gist of it and how you can also easily incorporate units regardless of their makeup, organization, equipment, or other quirks that are hard to account for in some systems.

EDIT 2: After a quick skim of the link @Redviiper provided, the rules I have in the PDF are quite different overall; I believe they're more comprehensive and useful and can more easily be adapted to our use for Modern Powers.

All right, so now that I'm home, let's do a quick overview of GURPS Mass Combat rules, and see how you guys like them.

From the introduction -

GURPS Mass Combat contains rules for the abstract resolution of battles between armies containing from hundreds to tens of thousands of warriors. Its focus is cinematic – it emphasizes dramatic clashes over the duller (if no less important) business of logistics and campaigning that often win real conflicts. Nevertheless, it does provide simplified rules for moving and supplying troops, and for raising and maintaining armies.

Seems to be suitable, I would think. Our business in the game is playing a nation, all aspects of it. Having extreme focus on combat, logistics, and supply makes the work disproportional for warfare as opposed to being at peace.

Each side in a battle consists of a number of elements, each of which is trained and equipped to fight in a particular fashion. Every element has a Troop Strength (TS) that reflects its size, type, and quality. For the purpose of resolving a battle, all elements on a side are combined into a single force, with a TS equal to the sum of its elements’ TS.

For our time period, an element would likely represent a squad of men (for a modern-day force), or a vehicle and its crew (such as a tank), or a heavy weapon, or an artillery piece and its crew.

Elements are the building blocks of a fighting force: soldiers, crewed vehicles, or weapons teams. Each element type has a name and is rated for Troop Strength (TS), special class (Class), transport weight (WT), mobility (Mob), cost to raise (Raise), and cost to maintain (Maintain).

You can also apply special features (such as night vision) to relevant units, or modify the troop or equipment quality level (for example, an army with good training, but poor equipment would have its TS modified appropriately).

Also included are costs and time for raising forces, as well as for logistic forces that accompany the army, costs for maintaining them, and other relevant factors that play into a long campaign. Reconnaissance contents occur when enemy forces are in the same area, and provide the opportunity for taking the initiative, surprising the enemy, or even ambushing them!

The combat itself is done by what's call a "quick contest" - both sides select a strategy in secret for a given round of fighting, reveal them, then roll 3d6 against a target number (let's say 15 for an elite general). You want to roll as low as possible here - what's relevant is your "margin of victory" in regards to your roll. So if my target number is 15, and I roll 9, that's a margin of victory of 6. Compare to my opponent's who let's say rolled a 10. His margin is five, meaning my margin is one better, so I win the round by one. The size of your victory as well as your tactic used determines the overall effects. A simple attack order will give you a little better position in subsequent rounds as well as a small casualty advantage (IE they take more casualties than you). A complex indirect attack might deal lots of casualties because of the risk it entails (you take a penalty to your target number, making success harder). A mobile defense lets you trade land for lives. And so on. These are all tactics that can be used in a battle resolution. If the numbers are public (and why wouldn't they be), two players can resolve this themselves without GM involvement as long as their can find either a third person willing to ensure there's no foul play going on, or they just trust each other to not lie regarding their choices each round.

I think that's enough to give an overview, but if someone would like me to go into more detail, please do so.