What is Fog of War?
Fog of War, also called dark chess, is chess in which you cannot see the whole board. You always see your own pieces and every square they could move to. An enemy piece standing on one of those squares is visible too; everything else is hidden under fog. As soon as your pieces move, the fog moves with them.
Because you may not know that your king is attacked, there is no check and no checkmate. You may move into an attack or leave your king where it can be taken, and the game is won by capturing the enemy king. Pieces move, capture and promote exactly as in standard chess, and the game starts from the usual position. It doesn't combine with Chess960, Three-Check or material odds.
Played online, each player sees only their own view, and spectators see no pieces until the game ends. Played by two people on one device, a curtain covers the board whenever the turn passes, so each player sees only their own side's view. When the game ends the fog lifts, and the replay shows every move.
Rule changes
| Rule | Standard Chess | Fog of War |
|---|---|---|
| What you see | Standard ChessThe whole board: every piece of both sides. | Fog of WarYour own pieces, every square they can move to, and any enemy piece on one of those squares. A pawn sees the empty squares ahead it can step to and the diagonals where it can capture; an enemy piece straight in front of it and an empty diagonal stay hidden. |
| The opponent's moves | Standard ChessSeen in full, on the board and in the move list. | Fog of WarHidden: the move list shows only that a move was made. If it took one of your pieces on a square you can no longer see, that square is marked. |
| Check | Standard ChessYou must get out of check and may never move into it. | Fog of WarThere is no check. You may move into an attack or leave your king attacked, and nobody warns you. |
| How to win | Standard ChessCheckmate the opponent's king. | Fog of WarCapture the opponent's king. The game ends at once, and the result says the king was captured. |
| Castling | Standard ChessThe squares between king and rook must be empty, and the king may not castle out of, through or into check. | Fog of WarThe squares between king and rook must still be empty, but castling out of, through or into an attacked square is allowed. |
| En passant | Standard ChessA pawn that has just moved two squares can be taken as if it had moved one. | Fog of WarThe same. When you can take en passant, the pawn that passed yours becomes visible, even if it stands in the fog. |
| Draws | Standard ChessStalemate, threefold repetition, the fifty-move rule and insufficient material. | Fog of WarStalemate (having no move at all, which is very rare without check), threefold repetition and the fifty-move rule. Lack of material never draws, because the kings can capture each other. |
History
Fog of War descends from Kriegspiel, a chess game of hidden information invented in 1899 by the English journalist Henry Michael Temple. In Kriegspiel each player sees only their own pieces, and a referee who follows the whole game on a third board tells them which of their attempted moves are legal.
Dark chess was devised in 1989 by Jens Bæk Nielsen and Torben Osted, who tried out changed Kriegspiel rules in a correspondence game. Its key idea, credited to Osted, is that a piece sees the squares it can move to, so no referee is needed: the board itself decides what each player knows. The aim became capturing the king, and a player is not told when their king is attacked.
Computers made the game easy to play, since they can hide and reveal squares for both sides at once. Under the name Fog of War, taken from the military phrase for the uncertainty of battle, it is now offered on large chess sites, which differ in small details such as what a pawn sees. The rules here follow that online form.