[Alexey Shirov would need to have seen ahead a mind-numbing 206 moves in order to be sure of forcing a win in a game at the World Cup.]
According to the rules of chess, a player can claim a draw if 50 moves pass (50 for each side) in which no piece has been captured and no pawn moved. The original idea of the rule was that if one side seems to have an advantage in the endgame (the stage of the game where only a few pieces or pawns remain), 50 moves should be more than sufficient to turn that advantage into checkmate. If there was no 50-move rule, one side could keep playing on in a theoretically drawn position hoping to win only because his opponent becomes fatigued. But leave it to computers to discover that there are certain winning endgames that, even with best play, require more than 50 moves to win. These endgames are incredibly rare, and seem more like composed problems than positions that might arise in actual play, and so the 50-move rule has been left enforce.
Now, incredibly, one of these endgames—two lone knights versus a lone rook and bishop—actually occurred in a very important game, in the semifinals of the World Cup. Alexey Shirov and Sergey Karjakin had tied their regular games in the semifinals. To break the tie, the two grandmasters played two rapid games, and it was the first of these that saw the unusual endgame. With best play (from the position below), computers tell us, Shirov as Black could have checkmated Karjakin in 208 moves!! Shirov didn’t see that, of course, and the game ended in a draw.
The excellent online daily newsletter Chess Today discusses the endgame in the December 12th issue. Chess Today is a great, subscription-based publication and well worth the modest fee.