I once did a Geeklist for BoardGame Geek about made-for-the-purpose introductory wargames.
Then, as now, I questioned whether these efforts ever really recruited many wargamers to the hobby.
The problem is that most of them are kind of like boiled cabbage. They've been so thoroughly cooked that they end up not having much flavor.
Most wargamers don't get their start with some simple game, I believe. Indeed, some people get their start with wildly inappropriate fare for beginners like ASL, Terrible Swift Sword or Empire III. Now, I'll admit that jumping right into ASL without any help is probably more likely to discourage than encourage a new player, one also doesn't need to start with Strike Force One, especially if you have a patient teacher.
And, in truth, most wargamers seem to get into the hobby with a buddy or two. Sometimes that friends is already a wargamer. Sometimes they're not, but the buddies learn their first games together. The social support is critical. And it's even somewhat amusing to play a few times, each time developing more understanding as you figure out rules you played wrong, or overlooked or simply didn't understand.
I remember that when my best friend and I played Stalingrad back in the 1970s it was a great revelation when we discovered the benefits of a continuous line! In our first couple of games we had stacked both armies to the maximum of three units per "square." The result was rather more like a Napoleonic or even ancient battle as the two armies tried to outflank each other and cut off supply. This naturally worked against the Soviets, who had the smaller army. Then one of us -- I like to think it was me, but I can't be sure after so much time has passed -- discovered that covering the whole front with a thinner line made grand encirclements less likely and we started having a World War II-style front.
It seems all so obvious now, but there wasn't a lot of help yet. Little bands of players were discovering this strange new hobby in isolation for the most part. It was fun, though.
What we didn't have, though, was a game designed to be introductory. Most wargames of the time would be considered pretty simple by today's standards, but that doesn't take into account how unprecedented they were, so they were not all that simple to us.
Probably the single most important tactic for making a game accessible to new players is having copious and well-illustrated examples of play. The old SPI introductory wargame Napoleon at Waterloo included a letter from Jim Dunnigan explaining the hobby, but more importantly, the back of the letter had a bunch of examples of combat, invaluable to someone who's never seen a wargame before.
Battle for Moscow has simple rules and is available online for free. But the original free GDW edition included a couple of examples of play covering movement and combat.
One of the biggest flaws in most magazine issue games is the lack of play examples. This is especially unfortunate when the game introduces some novel design element.
One of the nice things about many of the new games lately is the attention being paid to including a lot of examples. In some cases the examples can take up as much space in the rule book as the actual rules. This, more than just simple rules, is the most important thing making a game accessible.