The Game of Kriegspiel
Kriegspiel is variation of regular chess in which each of the two players may only
see his own pieces on his own chessboard.
A referee can see both sides, and maintains a third chessboard with the complete game,
ensuring that the players follow the rules.
Occasionally, a player will attempt an 
, not being aware of the position
of his opponent's pieces, and the player
must find a legal alternative.
As in chess, the objective is to capture the opponent's King.
This game was said to be popular amongst German generals as a relaxation during
the Second World War. Kriegspiel means "War Game",
and it is easy to understand the parallels. In Chess, there is certainty about both players' positions,
the pieces at their disposal, and the moves available to them.
So theoretically it should be possible, given enough time and black coffee,
to compute the optimum move for each player.
In practice of course, only Grand Masters can do this, and then not
perfectly otherwise there would be no point to the game.
In Kriegspiel, in contrast, certainty evaporates.
After a few moves, there can only be a hazy notion of where the enemy is to be found.
When pieces are taken, the winning player does not know what piece he has taken,
and the losing player does not know what has taken him, so the pieces available
to the opponent are no longer known. Some information about
the enemy can be deduced by covert reconnaisance
(attempting moves which prove to be illegal), but reconnaisance patrols may be
discovered when they become inadvertently engaged in conflict.
Surprise attacks and ambushes become a possibility.
Curiously, the game is in some ways easy for the relative beginner at chess to enjoy:
the genesis of this program was the discovery that my then 11-year old son and
his friends found the game intriguing and hilarious, but would end in frustration
when the referee failed to keep his chessboard in step, and could not replay from
memory the entire sequence of moves from beginning to end. I quickly learned to
pity the junior officer who would try to referee his Generals.
This computer version of the referee should avoid the likelihood of being sent to
the Eastern front...
