Kriegspiel (German for war game) is a chess variant invented by Henry Michael Temple in 1899 and based upon the original Kriegsspiel developed by Georg von Rassewitz in 1812. In this game each player can see their own pieces, but not those of their opponent. For this reason, it is necessary to have a third person (or computer) act as a referee, with full information about the progress of the game. When it is a player's turn he will attempt a move, which the referee will declare to be 'legal' or 'illegal'. If the move is illegal, the player tries again; if it is legal, that move stands. Each player is given information about checks and captures. They may also ask the referee if there are any legal captures with a pawn. Since the position of the opponent's pieces is unknown, Kriegspiel is not a game with perfect information. Chess Kriegspiel derives from a war game which was used in 19th century Germany to train military officers.
The game is played with three boards, one for each player and one for the umpire (and spectators). Each opponent knows the exact position only of his own pieces and does not know where the opponent's pieces are (but can keep track of how many there are). Only the umpire knows the exact current position of the game. The game proceeds in the following way. The umpire announces:
- White (or black) to move.
- Pawn tries, when it is possible for one's pawn to capture an opponent's pawn or piece. The umpire also indicates the square on which the capture is possible to the player who can make the capture. This gives extra information, but saves both players the bother of beginning every turn by trying all possible pawn captures. This is possible at no risk because pawns do not move the same way they capture. Hence, if no capture is possible, then the move is illegal and there is no penalty for attempting illegal moves. A pawn try is not announced if the pawn is pinned; that is, if completing the capture would expose the king to check. En-passant pawn tries are announced, but not the fact that they are en-passant captures.
- Pawn gone, when a pawn is captured.
- Piece gone, when a piece is captured.
- No, when the attempted move is illegal, given the opponent's position. For example: moving the king into check; moving a queen, rook, bishop, or pawn through squares occupied by the opponent's pieces; advancing a pawn into a square occupied by the opponent's pieces.
- Hell no (or Impossible), when the attempted move is always illegal regardless of the opponent's position. For example, moving a bishop a knight's move.
- Check on the vertical.
- Check on the horizontal.
- Check on the long diagonal (the longer of the two diagonals, from the king's point of view).
- Check on the short diagonal.
- Check by a knight.
- Checkmate, stalemate, draw by repetition, draw by insufficient force, 50-move draw.
Pawn promotions are not announced.
Kriegspiel problems 
The Problemist 1976
Kriegspiel is sometimes used in chess problems. In these, usual variations introduced by different black moves are replaced by variations introduced by different announcements.
An example of a Kriegspiel problem is shown at the right. White must checkmate Black in 8 moves, no matter where the black bishop initially is (it is somewhere on dark squares) and no matter what black plays. (Note that in a real game, black would not see white's moves, but for a problem in which white is to force a win, one must assume the worst-case scenario in which black guesses correctly on each move.) For example, 1. Ra1?? is a draw by stalemate if the black bishop was initially on a1. 1. Nf2 Bxf2 2. Kxf2 (or Rxf2) is stalemate as well. So, white should not move either the knight or the bishop, because either might capture the black bishop by accident. For the same reason, the white rook should move only to light squares -- but only half of the light squares are reachable without visiting a dark square along the way.
The solution is the following. White tries to play 1. Rg2.
- If this move is not possible (umpire says No), then the black bishop must be on b2, d2 or f2. In this case white can instead play 1. N(x)f2# (checkmate).
- If the move is possible, it is made and then black moves the bishop. White still doesn't know where the bishop is.
White continues with 2. Rg8.
- If not possible, then black bishop is on g3, g5 or g7. White plays 2. Be5. If black now plays 2...Bxe5, 3.Nf2#. Otherwise (any move by black) 3. Nf2+ Bxf2 4. Rxh2#.
- If possible, white continues 3. Rh8. (This is safe -- the black bishop can't be on h8 to be captured, because it wasn't on g7 on the previous turn.) 4. Rh5 5. Rb5 (if not possible, 5. Rh3 and 6.Be5). 6. Rb1 7. Nf2+ Bxf2 8. Kxf2#.
Rule variations 
Frankenstein suggested in 1903 a variation of the game where one player sees the board and another plays Kriegspiel. To make the game fair, the first player has to play with fewer pieces. Frankenstein proposed two variants:
- Pickle pot - the player who sees the board plays only with queen and bishop (as well as with king and 8 pawns in usual starting position).
- One-eye - same as above, but only with two rooks and bishop.
In both versions, it should be announced, which bishop remains (on c or f-file).
The Semi-kriegspiel, suggested by David Silverman in 1971 is similar to variations above. In this game the sighted side has only king and queen, which he/she can place on any legal square before the beginning of the game. In Modern kriegspiel by Bruce Trone (1986), after each move the player calls 7 squares, which must be opened by umpire. Otherwise the rules are as in usual kriegspiel.
See also 
- Kriegspiel by Hans L. Bodlaender.
- SchemingMind.com Internet server to play Kriegspiel.
- Berkeley Kriegspiel Home, Kriegspiel in Artificial intelligence research.
- Bologna Kriegspiel Home A page on Kriegspiel research
- U. Maryland Kriegspiel Home A page on Kriegspiel research
- Freeware Windows Kriegspiel game (called SearchAndDestroyChess).