whatever black's reply is going to be to Bxh7+, e6! wins the game for white.

Am I missing something?

and figured if ... KxF7, Ng5+ with mate to follow as the queen comes roaring out for the kill. I didn't spot that Black had any defense after Ng5+, as <apple_pi> points out. So that's my first mistake.

Second, I couldn't see much of a way forward for white if Black simply declines the sacrifice with 18... Kh8. Black seemed to have significant defensive resources in g6, Qg4, Nf5 and so fourth. So, so far as I could figure, Bxh7 Kh8 led to white winning a pawn only.

Huebner vs Korchnoi, 1987 (18.?)

White to play and win.

Material: B for N. The Black Kg8 has 1 legal move. The White Bc2 attacks h7, which the White Nf3 can reach in one move. The Black Bh5 pins Nf3 to Qd1, giving the opportunity for a discovered attack, as always (from an optimist’s perspective). The White Bc1 is available to control dark squares within the Black K-position. The White Rs and Qd1 require activation, which on the open board, will come rapidly if White gains the initiative. The White Kg1 is secure from check.

Candidates (18.): Bxh7+, e6

Black is better after

18.Bxh7 Kxh7 19.Ng5+ Kg6 20.Qd3+ Qf5

so White should activate his pieces before the obvious sacrifice Bxh7+.

White to play (18.?) "Difficult" (3 stars)

Well, the first comment I have to make is... wow, Korchnoi in trouble after only 18 moves?! I suppose it happens to the best of them. (I seem to remember a game Karpov had to resign after nine moves or so, because he overlooked a simple tactic, a double attack).

Candidate moves: Bxh7+, e6

I figured there must be an attack with the combination of Bxh7+ and Ng5+ (and either Qxh5 or an attack on the king moving forward to defend the bishop), or maybe e5-e6 first, followed by the above.

18. e6 This weakens the king position and cuts off the Black queens defense. Now:

A) 18...Bxf3 This gets rid of a key attacker, avoiding a future knight check on g5 after the bishop sac. Now 19. Qd3 Be4 (forced) 20. Rxe4 (only try for an advantage) dxe4! (20...fxe6 21. Rh4 Nf5 is also better for Black) 21. Qxd7 (forced, since 21. Qxe4?? Qd1+ mates in two more moves) Rxd7 22. exd7 Nxc3 23. Bb2 Ned5! 24. Re1 Rd8 25. Bxe4 Nxe4 26. Rxe4 Kf8 and after ...Rxd7 Black will be a solid pawn up in a winning endgame. But White can try

19. Bxh7+! Now:

A1) 19...Kh8 20. Qxf3 fxe6 21. Qh5 with a winning attack.

A2) 19...Kxh7? 20. Qd3+! Be4 21. Qh3+ followed by 22. exd7, winning the queen.

B) 18...fxe6! 19. Bxh7+ and now:

B1) 19...Kxh7 20. Ng5+ Kg6 (or 20...Kg8 21. Qxh5 with a winning attack, or 20...Kh6 21. Nf7+! Kg6 (21...Kh7 22. Qxh5+ Kg8 23. Ng5 transposes to 20...Kg8 line) 22. Ne5+ Kf6 23. Qxh5! (stronger than taking the queen because of the mate threat) g6 24. Qh6! Qd6 (God save the queen) 25. Bg5+ Kf5 26. g4# - but not the king!

B2) 19...Kh8! I can't find a follow-up here. So 18. Bxh7+ must come first!

~~~~~~~~~`

18. Bxh7+! Now 18...Kxh7 19. e6! transposes to lines above, so all that is left to look at is

18...Kh8 19. Bc2! Nxc3 (everything else leave Black a pawn down with no compensation) 20. Qd3 Ne4 (forced) 21. Ng5 Nxg5 (or 21...Bg6 22. e6! followed by 23. Qh3+ and attack continues - see transposition below) 22. Bxg5 Bg6 23. e6! Bxd3 (or 23...fxe6 24. Qh3+ Kg8 25. Nxe6 Bf5 26. Nxf8 Bxh3 27. Nxd7 Bxd7 and White has won an exchange) 24. exd7 Bxc2 25. Bxe7 and wins.

I did not understand this game. I saw 18. BxP/h7+, KxB/h7; 19.e6, PxP; 20.Ng5+, Kg6; but did not consider this a completely winning line.

Best Regards,
dh>

Sorry for the late answer, I think that after 21... Rf6 22.Qh7+ Kf8 23.Qh8+ Ng8 24.Nh7+ Kf7 25.Nxf6 Nxf6 26.Qh3 White is an exchange up and should win the game.

<Clearly, 19...fxe6 is bad because of 20 Ng5+ Kg8 21 Qxh5 or 20...Kg6 21 g4, with a winning position in either case.

Black's intention when analyzing the position at home was to insert the zwischenzug 19...Bxf3. Then 20 Qxf3 would allow him to play 20...fxe6 safely, with a fine game.

The crucial line occurs, of course, when White answers 19...Bxf3 with something more forceful, and that's why Korchnoi planned on 20 Qc2+ Be4 21 Rxe4 and now 21...fxe6! Despite the availibility of any number of discovered checks, Black's position is safe and quite sound.

But while studying the position for 50 minutes at the board, Black suddenly realized that White has a choice at move 20. He could play 20 Qd3! instead of 20 Qc2+, with the significant difference that 20...Be4 allows 21 Qh3+, followed by winning the queen. So...

19...resigns.>

However, it's not so easy after 18. Bh7+!! Kh8 when White must find 19. Bc2! Nxc3 20. Qd3 Ne4 21. Ng5 Qf5 22. Nxe4 dxe4 23. Qxe4 Qxe4 24. Bxe4 ± to +- (+1.42 @ 34 depth, Stockfish 8) to go a pawn up and maintain winning chances.

"Well played," he said.

As he was signing his scoresheet he said, "It's a pity you couldn't play like this 7 years ago."

2) +5.23 (31 ply) 19...Qc6 20.Ng5+ Kg6 21.g4 f5 22.gxh5+ Kh6 23.Qf3 Rde8 24.Bf4 Rf6 25.Rac1 Nb6 26.Qh3 Ref8 27.Nf7+ Kh7 28.h6 R8xf7 29.exf7 Nbc8 30.f8=R Rxf8 31.hxg7+ Kxg7 32.Be5+ Kg8 33.Kh1 Kf7 34.Qh7+ Ke6 35.Bg7+ Kf7 36.f3 Qg6 37.Qxg6+ Nxg6 38.Bxf8 Nxf8 39.Re5 Nd6 40.Rxd5 Kf6 41.Kg2 Ne6 42.Kg3 Kg5 43.h4+ Kg6 44.Rd2