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> Do you have something in mind?
-Not a wargame, but multiplayer Go exists and wouldn't be hard to implement. Boards can even be edited in MS Paint and play can continue quite smoothly if some player gives up.
-Some Risk variant -very simple and it's trivially easy to adjust to whichever player count there is.
-Supremacy, which is sort of a more complex and geopolitical Risk with markets, resources and the like. Made for more than 2 players, but still a fixed number of them. But of course, adjustments are always possible.
-Axis&Allies. On the plus side it already has rules for what happens if the number of players is less than 5.
-After the Holocaust, a kc tier economic simulator (with some military mechanics) set in post-apocalyptic America and designed for 4 players. Requires a higher commitment, but looks fun.
-Tetrarchia: 4-player game set in the late Roman Empire. Supposedly simple. I know little about it, though.
I briefly thought of Diplomacy (in its Gunboat variant -no communication allowed between players, only the laconic language of moving armies), but it's designed around simultaneous action and that's hard to implement in an imageboard or redesign.
Otherwise, most wargames are strictly two-sided. A few are designed for two teams of players (e.g. there's a kc tyre North African Campaign wargame where players must keep track of everything down to each individual plane), but those are too complex and unwieldy for playing here. This leaves us with a large pool of games for 2 players. Several arrangements are possible for playing them with more than two people: one player with one side and everyone else playing the other side, several concurrent games or team play. The last is unwieldy.
Out of these, I can vouch for the two I've already mentioned ITT -Eastern Front 2 and Battle for Moscow. Boardgamegeek has several interesting possibilities I know little about it, such as Hannibal:Rome vs. Carthage, Twilight Struggle (Cold War), Napoleon:The Waterloo Campaign, but those have deeper rules and require more player commitment.
A lot of those games are set in the World Wars, so finding one without emotional baggage is harder. But there are a lot set in Antiquity, the Napoleonic Wars and other settings.
Helper programs also exist for wargames, such as VASSAL. But depending on how simple the game is, image editing software may be enough.