I think @unmanneddrone may have mentioned the game once, but Unity of Command is a pretty cool thing and just ended its Steam debut sale. The game came out last fall but only just now showed up on Steam (and thus I only just now became aware of the game).
It’s pretty much the most accessible classic-style wargame I’ve encountered. The closest thing to an operation-level wargame I’ve ever touched is Advanced Wars, and my experience with that actually helped me slide into Unity. There are huge differences of course but the basement movement and turn interface works the same way, and the game manages to be addictive in the same fashion. Even with Borderlands 2, Dishonored, and now Zerro Escape Virtue’s Last Reward on my plate, Unity still manages to cut an hour away from my time.
On the accessibility angle, the game will also run on pretty damn much anything. If you have a 2GHz processor and a couple gigs of RAM you’re good to go, even on integrated graphics, and the game manages to look great due to pretty slick art direction. PC or Mac.
At the very least keep an eye on this one for the fall Steam Sale.