The rarest win in chess is arguably the two knights checkmate, though whether it is a "win" is debatable.
Why Two Knights Checkmate is Rare
Several factors contribute to the rarity of this "win":
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Insufficient Material: With only two knights and a king against a king, a forced checkmate is generally impossible if the defending king plays optimally. The attacking side doesn't have enough material to force the opponent into checkmate.
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Requires Opponent's Help (or Blunder): To achieve a checkmate with two knights, the defending king must move into a position where it's either checkmated or stalemated in the next move. This typically only happens if the defending player makes a significant blunder or misplays the position. The defending king must essentially walk into the checkmate.
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Stalemate Risk: Often, the attacking side can inadvertently stalemate the defending king while trying to maneuver their knights. Stalemate results in a draw, not a win.
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50-Move Rule: Even if the attacking side is theoretically able to force the defending king into a position for a two-knights checkmate, the 50-move rule often comes into play. If 50 moves pass without a pawn move or a capture, the game is declared a draw. This makes executing a two-knights checkmate within the required timeframe incredibly difficult.
Is it Really a Win?
It's important to note that achieving a two-knights checkmate is often considered more of a result of the opponent's mistake than skillful play by the attacking side. Because the opponent has to blunder, it's not always considered a "legitimate" win, in the same way as a checkmate achieved through superior tactics and strategy. Some consider it a win due to the opponent resigning because they do not see a path to avoiding the forced stalemate/checkmate.
Other Rare Chess Outcomes
While the two-knights checkmate is extremely rare, other chess outcomes are also uncommon:
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Fool's Mate (Fastest Checkmate): Though theoretically possible in some games, the Fool's Mate (1. f3 e5 2. g4 Qh4#) is rare in high-level play because it relies on extremely weak opening moves.
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Smothered Mate: A checkmate where the king is surrounded by its own pieces and unable to move. While not as rare as a two-knights checkmate, they require specific board configurations and tactical awareness.
Conclusion
In conclusion, while other unusual outcomes exist, the two knights checkmate is arguably the rarest "win" in chess, due to the need for opponent cooperation (or blunder), the risk of stalemate, and the limitations imposed by the 50-move rule.
