“You must take your opponent into a deep dark forest where 2+2=5, and the path leading out is only wide enough for one." – Michail Tal

I am looking for ways to systematically find lines where one player is forced to find series of precise moves.


The battle of Thermopylae was fought in 480 BC between the Achaemenid Persian Empire under Xerxes I. and an alliance of Greek city-states led by Sparta under Leonidas I.

During two full days of battle, the small force led by Leonidas blocked the only road by which the massive Persian army could traverse the narrow pass.

The performance of the Greek defenders is well known as an example of the advantages of training, equipment, and use of terrain as force multipliers.


In chess, I consider a 'thermopylae' to be an only-move that must be found by ones opponent in order to not blunder away their game. Similar to the historic example, it is a narrow pass, an only road, and when studied and prepared at home, it gives rise to a possible advantage through training, equipment, and use of terrain. (You get the point...)

Example 1

Consider the equal position given below where Qd1 is a thermopylae from blacks view since both Qd2 and Qd3 (and every other move of course) lead to a significant advantage for black.

[fen "rqr3k1/3n1ppp/bp2pb2/1N1p4/PP1Q1P2/2P1BR2/4B1PP/R5K1 w - - 0 1"]

Example 2

In the equal position below, white needs to find Be4 to not blunder away the game. If this is found, the game 'widens' after Ke6, but remains 'narrow' after Nb4, asking again for precise moves from white.

[fen "r4r2/5k2/3p4/2pn4/2R5/p7/2B2PPP/1R4K1 w - - 0 35"]


Is there a way to (have engines) systematically go through tabiae (the ends of opening lines) and extract lines that lead to such thermopylaes, possibly a multitude of these down a certain line? (I understand this will include a number of trivial recaptures and forced moves, but I'll go with that.)

  • I'd appreciate an explanation to why the down-vote - it could help me improve later questions in general, and this one in particular. Commented Jun 11, 2022 at 19:08
  • What's the quote? A decreasing amount of moves is a sign of a deteriorating position. If your opponent doesn't have good moves left, then they only have bad moves. In the opening, this doesn't happen because both players have a variety of options.
    – DdogBoss
    Commented Jun 11, 2022 at 19:42
  • 4
    I downvote also because your example, in comparison with what Tal was talking about, is trivial and makes it quite clear, I regret to say, that you have little idea how chess is played at a level higher than elementary. You seem to believe that you have discovered a new strategic concept but you have not, and your attempt to dress it in intellectual flummery is distasteful. i am being plainspoken only because you did ask for clarification..
    – Philip Roe
    Commented Jun 11, 2022 at 23:53
  • 2
    Doesn't basically any tactics quiz provide an answer to this question? Commented Jun 13, 2022 at 15:11
  • 1
    On go through tabiae (the ends of opening lines) and extract lines that lead to such thermopylaes so you have an opening line of say 10 plies, you want to find a continuation line such that your opponent will be brought to a series of positions where there is only one playable move (critical position). And in doing so you are willing to give up a pawn or two worth of evaluation or even a whole piece just to bring your opponent into a series of thermopylae states. If the opponent happens to find all the best moves, you will lose the game - the payment.
    – ferdy
    Commented Jun 14, 2022 at 12:53

1 Answer 1


Your concept of "Thermopylae" is just what chess player call a "forced move". The problem is, smost forced moves are trivial (for example, recaptures or moving out of check), while others can be hard to find. The number of alternatives does not always determine the difficulty of a position.

Engines are not designed to estimate how hard to find a move is for a human. You could certainly try to tweak the evaluation function so that lines containing lots of forced moves get a bonus, but this doesn't mean the new engine will be harder to play against. Rather, it'd be an engine that likes playing some sort of "hope chess".

As Federico Poloni suggests on his comment, every puzzle collection is a collection of "Thermopylae", so sites like Lichess that generate puzzle based on games played by their users are certainly doing a scan of this sort.

Still, the vast majority of those puzzles are terribly easy, so it looks like this method wouldn't be particularly useful for opening preparation, specially if we take into account that current opening theory will hardly ever end right before a hard-to-find forced move. After all, who would have a line on his repertoire that requires him to find an outstanding move on the spot or else get a lost position?

  • "Still, the vast majority of those puzzles are terribly easy" - not at all. There are puzzles on these sites that can stump some of the best tactical players in the world (e.g. Nakamura, on Chess.com) for near-on hours, even though they know it's a puzzle. Pose a series of those, midgame, and you'll almost surely win. It's a highly non-trivial question, though where I agree with you is that creating forcing positions and positions which are forcing & difficult are not the same. Commented Jun 16, 2022 at 21:57
  • See also: "(I understand this will include a number of trivial recaptures... but I'll go with that.)". Not the answer I am looking for. Commented Jun 16, 2022 at 22:23
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    @MobeusZoom That's why I said "the vast majority" and not "every single one". If you pick a lichess puzzle at random, there is a very little chance of it being one of those that makes Nakamura struggle. The reason why you can get a collection of hard puzzles is that there are thousands of humans playing the puzzles over and over again to determine their difficulty. That does not apply to this question.
    – David
    Commented Jun 17, 2022 at 11:38
  • @PeterFischer If you don't mind having a 99.9%+ of trivial moves, then yes. That way exists.
    – David
    Commented Jun 17, 2022 at 11:39

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