Example D6 Combat System

Combat systems in games have to be more than effective. They have to be fun. The old "classic grognard" wargame was about nothing but combat, and the systems that evolved became so overburdened by tiny details that the fun was slowly leeched from them. So you can have a system that resolves combat "like it should" (i.e. elites not being taken down by greens), which is totally un-fun.

I'd look more at what you want combat to "feel" like. Do you want the units to be tough, making combats a slow game of attrition, or fragile, which emphasizes maneuver and position? Are the most experienced units very much more effective, or does the technology of every unit pretty much even out the field? This will count a lot toward conserving units, as beneficial experience won't be squandered if it pays to save it up.

In more concrete terms, begin with your basic system. Since your rolls are boolean rather than numeric (a 6 is "1", and anything else is "0"), there's no theoretical limit on how long a combat can continue. Every roll to hit can come up all misses, even against units without defenses. This can continue forever unless you rule it out somehow, either by decree ("Combat is over the course of three rounds"), or by adding some other variable that will tip the balance one way or another over the course of time.

I think the issue of "endless combat" is only a problem if each round is the same as the last except for the number of dice being rolled. Basically, are there enough tough decisions the players need to make each turn? Players should be presented with options that are painful to choose between. If there is always a best-case option, the game isn't going to be very much fun. These choices should, ideally, lead to the conditions that break the infinite loop.

So there has to be some other element, like a resource that is allocated, that builds over time into a combat-significant effect. Or perhaps movement options influence combat, meaning the players will keep changing their deployments to gain advantages. In most cases, having a option that can be applied to more than one unit, or now instead of later, will force the player to choose. In an open system like yours, there's no reason why a player can't have numerous factors come toether momentarily to make a given unit invincible. If the condition is a matter of numerous choices rather than dice-rolls, the players will see it as fair.