citação:
Originalmente colocada por zerorpm
The game ends when one of the players captures his opponent's king, when one of the player's resigns or there is a stalemate.
When a player's king is threatened by an opposing piece, it is said to be "in check". When a player places the opposing king
in check he should announce, "check". The object of a player is not merely to place his opponent's king in check but to make certain that every square where the king has a possibility of movement is also covered. This is called checkmate. The king is considered captured.
http://www.conservativebookstore.com/chess/end.htm
Aliás a regra de obrigatoriedade é lógica porque, caso contrário o jogo finda com movimentos ainda possíveis. Assim como não é possível movimentar o rei ou alguma peça que ponha o próprio rei em cheque. O xadrez é um jogo de cavalheiros e em nada se parece com o nosso parlamento...lol
Moving from a checked position to another checked position
Playing chess with a friend, I put his King in check with my rook. He then moved his King once to the right and captured my pawn, however, he did not realize that in making that move, he was vulnerable to my queen, so I captured his King. Now, another friend that was watching said that he could not make that move because he put his King in check and that is not allowed. I thought and still think, that if I put him in check and he makes a move that doesn't take him out of check, then I can capture his King. I always thought that the ruling involving a King being in check was that if a King is NOT in check, he cannot make a move to put himself in check, however, if he is in check and makes a move that keeps him in check, then I can capture.
Your friend was right. Chess is not won by capturing kings but by mating the opponent. If a player makes an illegal move, i.e., one that puts his king in check, he must make another move, regardless if he was in check or not. (The `touched piece' rule applies here too!)
Of course, when a player is in check, and all moves lead to a position where he still is in check, then he is mated, so he lost; and when a player is not in check but all moves lead to a position where he is in check, he is stalemated and the game is a draw.
http://www.chessvariants.com/d.chess/faq.html