Are you looking for a board game sort of wargame, or a tabletop miniatures wargame?
I've heard that Memoir is one of the more simplistic versions of that game system; Battlelore or Command and Colors Ancients sound like they'd have more depth while still being accessible. But I haven't played any of those games. And CC:A uses blocks with stickers instead of miniature units, which might be a visual turnoff.
Board Game Geek might be a good place to look for ideas - just in case you're not looking there already.
I was thinking about getting a wargame recently (I've pretty much settled on Tigris and Euphrates instead) and CC:A (mentioned above), Combat Commander, and Conflict of Heroes. They're all highly rated games and seemed to gain lots of praise for being accessible. I haven't played any of them, though.
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