My concept is to have the players be part of a larger combat force involved in the D-Day landing in Normandy. The players would engage in standard combat but the larger battle waging around them would be fought using mass combat rules (otherwise it would be hopelessly bogged down by the minutiae of standard combat rules). I intend for the game to be fun enough that good players re-join if their characters are killed early on; and I want the results of the large-scale combats to vary from game to game so it doesn't feel like a closely followed script.
I want the efforts of the PC's to affect the outcome of the larger-scale battles and vice versa (where it makes sense to).
At some point I intend to have the characters involved in vehicle combat (esp. tanks) as well; either as supporting infantry, engineers, drivers or gunners and would like to find a ruleset that is comprehensive enough to allow for clever tank-to-tank combat. A system that takes into account the weaknesses and strengths of Axis and Allied tanks would be preferred though I suppose anything can be tweaked.